Current Biology, Vol. 15, 1–7, June 7, 2005, ©2005 Elsevier Ltd All rights reserved. DOI 10.1016/j.cub.2005.05.020
A Small-Molecule Inhibitor of Mps1
Blocks the Spindle-Checkpoint Response
to a Lack of Tension on Mitotic Chromosomes
Russell K. Dorer,1,2,4,7Sheng Zhong,6,8
John A. Tallarico,4Wing Hung Wong,3,6,8
Timothy J. Mitchison,4,5and Andrew W. Murray1,2,*
1Department of Molecular and Cellular Biology
2Bauer Center for Genomics Research
3Department of Statistics
Cambridge, Massachusetts 02138
4Institute of Chemistry and Cell Biology
5Department of Systems Biology
250 Longwood Avenue
Harvard Medical School
Boston, Massachusetts 02115
6Department of Biostatistics
Harvard School of Public Health
Boston, Massachusetts 02115
minichromosomes (10–15 kb) segregate poorly in mito-
sis, activate the spindle checkpoint, and have a much
higher loss frequency than their circular counterparts
or natural chromosomes . The CDC28-VF mutation
lies in the critical cyclin-dependent kinase that drives
cells into mitosis and extends this checkpoint-depen-
dent delay and prevents colonies from forming on solid
medium. We used a strain containing the CDC28-VF
mutation, a linear minichromosome, and PTET-CDC20-
127, a dominant, checkpoint-inhibitory allele of CDC20
(an essential activator of the ubiquitin-dependent pro-
teolysis that initiates chromosome segregation) under
the control of the tetracycline promoter . In the ab-
sence of doxycycline, CDC20-127 is expressed and the
checkpoint is inactivated, but in the presence of doxy-
cyline, CDC20-127 is repressed and the checkpoint is
restored. Thus, combining PTET-CDC20-127, CDC28-VF,
and a linear minichromosome creates a strain that can-
not grow in the presence of doxycycline because the
minichromosome activates the spindle checkpoint.
This strain allowed us to identify checkpoint inhibitors
that enable faster growth in the presence of doxycy-
cline. Because we screened for increased cell prolifera-
tion, our screen selected for a minimal level of specific-
ity: Compounds that strongly inhibit any of the more
than 1100 yeast proteins that are essential for viability
could not be recovered.
We screened 140,000 small molecules and identified
five compounds (from the Chembridge DIVERSet E)
that permitted proliferation to the same extent as dele-
tion of MAD2, a known spindle-checkpoint gene. We
resynthesized these compounds (see Supplemental
Experimental Procedures), tested intermediates in the
above-described minichromosome assay, and found a
single active compound common to all five reagents
(Figure 1A); we named this compound “cincreasin”
(chromosome instability increasing compound). Re-
lated compounds were less active (see Figure S1 in the
Supplemental Data available with this article online). A
dose-response assay showed that cincreasin activates
proliferation in the minichromosome assay at 30–40
?M, about 10-fold lower than the IC50for viability (Fig-
We tested whether cincreasin overcomes the spin-
dle-checkpoint arrest caused by disrupting the micro-
tubule-kinetochore attachments with the microtubule-
depolymerizing drug benomyl. The spindle checkpoint
arrests wild-type cells treated with 60 ?g/ml benomyl
in mitosis for several hours, and viability is maintained
[1, 2]. In this situation, cells deleted for BUB2 delay in
mitosis at the spindle checkpoint but gradually lose via-
bility because they eventually bud despite the improper
placement of the spindle [11–13]. Cells deleted for both
BUB2 and MAD2 do not delay when treated with micro-
tubule-depolymerizing drugs and rapidly lose viability.
We measured the viability and rate of rebudding of
bub2D in 60 ?g/ml benomyl plus DMSO, bub2D in 60
?g/ml benomyl plus 150 ?M cincreasin, and bub2D
mad2D in 60 ?g/ml benomyl plus DMSO. We found no
effect of cincreasin on either viability or rebudding in
The spindle checkpoint prevents chromosome loss
by preventing chromosome segregation in cells with
improperly attached chromosomes [1–3]. The check-
point senses defects in the attachment of chromo-
somes to the mitotic spindle  and the tension ex-
erted on chromosomes by spindle forces in mitosis
[5–7]. Because many cancers have defects in chro-
mosome segregation, this checkpoint may be re-
quired for survival of tumor cells and may be a target
for chemotherapy. We performed a phenotype-based
chemical-genetic screen in budding yeast and iden-
tified an inhibitor of the spindle checkpoint, called
cincreasin. We used a genome-wide collection of yeast
gene-deletion strains and traditional genetic and bio-
chemical analysis to show that the target of cincreasin
is Mps1, a protein kinase required for checkpoint func-
tion . Despite the requirement for Mps1 for sensing
both the lack of microtubule attachment and tension at
kinetochores, we find concentrations of cincreasin that
selectively inhibit the tension-sensitive branch of the
spindle checkpoint. At these concentrations, cincreasin
causes lethal chromosome missegregation in mutants
that display chromosomal instability. Our results de-
monstrate that Mps1 can be exploited as a target and
that inhibiting the tension-sensitive branch of the
spindle checkpoint may be a way of selectively killing
cancer cells that display chromosomal instability.
Results and Discussion
ASmall-MoleculeInhibitorof the Spindle-Checkpoint
Delay Caused by Kinetochore Tension Defects
We devised a screen in which molecules that inhibit the
spindle checkpoint stimulate cell proliferation. Linear
7Present address: Department of Pathology, Brigham and Wom-
en’s Hospital, Boston, Massachusetts 02115.
8Present address: Department of Statistics, Stanford University,
Sequoia Hall, 390 Serra Mall, Stanford, California 94305.
Figure 1. Cincreasin Inhibits the Tension-
Dependent Spindle Checkpoint but Does
Not Inhibit the Attachment-Sensitive Check-
(A) Cincreasin bypasses the growth delay
caused by linear minichromosomes. Five-
fold serial dilutions of YBS151 (CDC28VF tet-
CDC20-127+ SLC::LEU2) were spotted on
minimal plates containing 0.1% DMSO and
the indicated concentration of cincreasin
and then grown for 2 days at 30°C and pho-
(B) and (C) The mean plus standard deviation
of the viability and rate of rebudding of the
indicated bub2D strains treated with the
microtubule-depolymerizing drug benomyl
(60 ?g/ml). No effect of cincreasin on viabil-
ity or rebudding was found.
(D) Experimental design to evaluate the
checkpoint response to tension defects
(modified from ). Cdc6-depleted cells go
through mitosis without replicating their
chromosomes. Kinetochores that are not un-
der tension activate the spindle checkpoint
and stabilize Pds1 (gray). Inactivation of the
checkpoint by deletion of MAD2 or treatment
with cincreasin allows degradation of Pds1.
(E) Pds1p levels were monitored during the
cell cycle (as in ) in cells that were de-
pleted of Cdc6 (YBS420) and were com-
pared to the levels in Cdc6-depleted cells
that also lacked Mad2 (YBS514); see Sup-
plemental Experimental Procedures for de-
benomyl, indicating that cincreasin is unable to inhibit
the attachment-sensitive branch of the spindle check-
point (Figures 1B and C).
Tension is generated on kinetochores during mitosis
when the poleward microtubule-dependent forces ex-
erted on kinetochores are opposed by the linkage be-
tween sister chromatids [5–7]. Cells depleted for the
replication protein Cdc6 do not replicate their DNA, but
they still proceed through mitosis. However, tension
cannot be generated because the chromatids lack sis-
ters, and the spindle checkpoint is activated, stabilizing
Pds1p, a protein that inhibits sister-chromatid segrega-
tion and is a target of Cdc20-dependent ubiquitination
[7, 14]. We evaluated the effects of cincreasin on Pds1p
degradation in cells that lack Cdc6 (cdc6D GAL-CDC6)
compared to in Cdc6-depleted cells that lack Mad2
(cdc6D GAL-CDC6 mad2D, Figures 1D and 1E). Pds1p
levels were stabilized in control DMSO-treated cells
containing unreplicated DNA but not in cells treated
with 150 ?M cincreasin or in mad2D cells, indicating
that cincreasin keeps the spindle checkpoint from de-
laying the exit from mitosis in cells whose kinetochores
are not under tension.
Cincreasin Increases the Chromosome Loss Rate
in Wild-Type Cells and Checkpoint Mutants
Because cincreasin perturbs the response to chromo-
somes that are not under tension, we measured the
chromosome loss rate in wild-type yeast cells treated
with cincreasin by determining the loss rate of a nones-
sential test chromosome via a colony-color assay [15,
16]. The test strain harbors the ade2-101 (ochre) muta-
tion and a test chromosome bearing the SUP11 gene
(an ochre-suppressing tRNA), which is lost 100 times
more frequently than normal chromosomes. Losing the
test chromosome makes the cells red instead of white.
A Chemical Inhibitor of the Spindle Checkpoint
If this loss occurs in the first division, the resulting col-
ony is half red and half white, and the rate of chromo-
some missegregation is calculated by dividing the
number of half-red colonies by the total number of col-
onies containing the test chromosome (as in ). We
found that cincreasin increases the chromosome loss
rate of wild-type W303 cells in a dose-dependent man-
ner (Figure 2A), indicating that cincreasin strongly af-
fects the fidelity of chromosome segregation in other-
To determine whether the effect of cincreasin on
chromosome segregation can be explained solely by
inhibition of the spindle checkpoint, we measured the
loss rate in cells deleted for several checkpoint genes
and treated with cincreasin. We found that cincreasin
dramatically increased the loss rate in mad1D, mad2D,
and mad3D cells (10-fold, 20-fold, and 15-fold at 60,
60, and 120 ?M, respectively; Figure 2B); the loss rates
of mad1D and mad2D mutants at 120 ?M cincreasin
were too high to measure accurately. Because the ef-
fects of cincreasin on these checkpoint mutants are so
severe, we conclude that cincreasin must inhibit other
aspects of chromosome segregation in addition to the
spindle checkpoint and that the target of cincreasin
cannot solely be Mad1, Mad2, or Mad3.
A Genome-Wide Genetic Screen Reveals a Unique
Chemogenetic Sensitivity Profile for Cincreasin
Cincreasin might interfere with two aspects of mitosis,
the tension-sensitive branch of the spindle checkpoint,
and some other mitotic process. To investigate this is-
sue, we performed a genome-wide screen for deletion
mutants sensitive to cincreasin. We inoculated w4,700
haploid deletion mutants onto rich medium plus either
200 ?M cincreasin or DMSO and found 124 cincreasin-
sensitive strains. Of these 124 strains, 71 mutants
scored strongly (32; +++) or moderately (39; ++) sensi-
tive to cincreasin (Table S1). Sixteen of the 32 (50%)
strong and 7 of the 39 (18%) moderate mutants have
previously-reported defects in kinetochore structure
and function (mcm16D, mcm17D, mcm21D, mcm22D,
ctf19D, ctf3D, ctf4D, bim1D, chl1D, and sgo1D), micro-
tubule motors (cin8D and kar3D), microtubule stability
or structure (gim3D, gim4D, and gim5D), sister-chroma-
tid cohesion (dcc1D and ctf8D), the spindle checkpoint
(bub1D, bub3D, and sgo1D), or other aspects of mitosis
(cdh1D, rts1D, pac10D, and cik1D) (reviewed in ).
Representative mutants are shown (Figure 2C). The
strong sensitivity of bub1D and bub3D mutants excludes
Bub1 and Bub3 as the sole targets of cincreasin. Some
of the remaining deletion mutants (i.e., VMA8, VMA22,
LSM1, YGL072C, YDJ1, YEL059W, and RVS167) were
recently identified as sensitive to at least four of ten
compounds of diverse activities and structure and thus
may be multidrug resistant . Nevertheless, when
functionally analyzed with the gene ontogeny (GO) term
finder (www.yeastgenome.org), no other major cellular-
process categories outside of genes involved in chro-
mosome segregation are significantly enriched (data
not shown). Moreover, dam1-11, dam1-24, and dad1-1
mutants, critical for microtubule-kinetochore interac-
tions [19–21], are very cincreasin-sensitive (data not
shown). These data suggest two possibilities. Either the
Figure 2. Cincreasin Increases Chromosome Loss in Wild-Type and
The loss of a nonessential test chromosome was scored with a
colony-color assay to measure the effect of cincreasin on chromo-
some loss rate of wild-type and mad1D, mad2D, or mad3D mutants
(see Supplemental Experimental Procedures for details).
(A) Wild-type cells (YMB108) treated with the indicated concentra-
tion of cincreasin.
(B) Wild-type (YMB108) and madD mutants (YMB111, YMB113, and
YJR111) treated with DMSO, 60 ?M, or 120 ?M cincreasin.
(C) Mutants defective in chromosome segregation are extremely
sensitive to cincreasin. Ten-fold serial dilutions of the indicated
haploid deletion strain (isogenic with the wild-type control
[BY4741]) were spotted on YPD (rich media) + 0.1% DMSO or
YPD + 0.1% DMSO and 200 ?M cincreasin, grown 2 days at 30°C,
and photographed. The mutants shown were strongly sensitive to
cincreasin (+++, no growth), except mcm16D, which was moder-
ately sensitive (++, growth of the highest dilution only). In another
assay, the IC50for cin8D (approximately 40 ?M) is 10–20-fold lower
than for wild-type (data not shown).
Figure 3. Cincreasin Causes Missegregation of Chromosomes in cin8D Cells
(A) Haploid cin8D cells were released from α-factor arrest into media containing either DMSO or 120 ?M cincreasin. The percentage of large-
budded cells in which any two GFP-labeled sister chromatids separated was counted at the indicated time points. In DMSO, the majority of
chromatids do not separate because cells delay in metaphase at the spindle checkpoint.
(B) The percentage of cells in which both GFP-labeled chromosomes missegregated to the same pole.
(C) The percentages of cells with the indicated number of GFP-labeled chromosomes were measured 7.5 hr after release into media containing
cincreasin or DMSO (as in [A]).
(D) Schematic interpretation of this experiment. cin8D cells treated with cincreasin separated their chromosomes without a checkpoint-
dependent delay and often missegregated their chromosomes to the same pole, resulting in aneuploidy and death. DMSO-treated cells
delayed in metaphase at the checkpoint, corrected errors due to the cin8D mutation in segregation, and then segregated their chromo-
sensitivity of every mutant is due to cincreasin effects
on the spindle checkpoint, effects that make cells very
sensitive to a variety of spindle defects, or the sensitiv-
ity of some mutants is due to direct cincreasin effects
on spindle structure and function. We favor the latter.
Taken together, these data suggest that cincreasin in-
hibits microtubule and/or kinetochore function in addi-
tion to the spindle checkpoint.
Many cincreasin-sensitive mutants affect chromo-
some segregation and require the spindle checkpoint
for survival. For example, cin8D mutants, which lack a
microtubule motor, are synthetically lethal with dele-
tions of MAD1 or MAD2 . To test whether the check-
point causes a metaphase delay in cin8D mutants and
promotes proper chromosome segregation, cells with
green fluorescent protein (GFP)-labeled chromosomes
 were synchronized in G1 with α-factor and released
into media containing either DMSO (control) or cincrea-
sin. cin8D mutants treated with DMSO (control) delayed
in metaphase for 2–4 hr and then proceeded through
anaphase asynchronously (Figure 3A). However, when
treated with cincreasin, cin8D mutants began to sepa-
rate sister chromatids 90 min after release from G1 (Fig-
ure 3A). In addition, cincreasin caused massive chro-
mosome loss in cin8D cells (Figures 3A and 3B), which
produced many aneuploid cells after 7.5 hr (Figure 3C).
Thus the strong cincreasin sensitivity of cin8D mutants
is explained by massive chromosome missegregation
(summarized in Figure 3D), suggesting that in cin8D
mutants, some chromosomes are not under tension be-
cause both sister kinetochores have attached to the
same pole (mono-orientation), thus activating the
checkpoint. The high fraction of cells that complete mi-
tosis with both copies of the labeled chromosome in a
single daughter cell can be explained in two ways: The
absence of Cin8 causes most chromosomes to mono-
orient and the checkpoint holds cells in prometaphase
while this defect is corrected, or cincreasin increases
the probability of mono-orientation in cin8D cells, as
well as inhibiting the checkpoint.
Target Identification with Drug-Induced
In general, it is hard to identify the target of a chemical
inhibitor found in phenotype-based screens. In mam-
malian cells, most approaches are difficult and require
a high affinity for the target . In yeast, there is a
genetic approach, which takes advantage of the fact
that a heterozygous strain deleted for one of the two
copies of a drug’s target typically expresses less of the
target and is hypersensitive to the drug [25–27]. We
performed a genome-wide screen of diploid cells con-
taining heterozygous gene deletions to identify poten-
tial targets of cincreasin. A simple small molecule like
cincreasin may bind with low affinity to many proteins,
so we expected to find several potential targets. Be-
cause the growth assays are competitive, fitness profil-
ing with heterozygotes is very sensitive to subtle differ-
ences in relative growth rates and, by extrapolation, to
We used a collection of w5900 yeast strains with a
heterozygous deletion marked with two unique, 20 bp,
oligonucleotide molecular bar codes. The relative abun-
dance of each strain is measured by amplifying the bar
A Chemical Inhibitor of the Spindle Checkpoint
Figure 4. Mps1 Is a Target of Cincreasin
(A) MPS1/mps1D heterozygotes are sensi-
tive to cincreasin when grown in competition
with wild-type yeast in the absence of any
other perturbation. After 20 generations, the
MPS1/mps1D strain was 50-fold less abun-
dant than at time zero.
(B) The IPL1/ipl1D heterozygous deletion
strain is only slightly sensitive to 200 ?M
cincreasin when grown in competition with
wild-type yeast. Viability assays were per-
formed in triplicate, and the mean plus stan-
dard deviation of the ratio of the indicated
strains is plotted.
(C and D) Mps1 is an in vitro target of
cincreasin. Mps1 kinase (Mps1-GST) was
assayed for its ability to phosphorylate my-
elin basic protein in vitro in the presence of
the indicated concentrations of cincreasin.
Kinase assays were performed in triplicate,
and the mean plus standard deviation of the
image density (determined with a phospho-
imager) is plotted in (D). Similar results were
obtained for the autophosphorylation activ-
ity of Mps1 (Figure S4).
(E) The MPS1-T602S kinase-domain muta-
tion confers resistance to cincreasin. Five-
fold serial dilutions of isogenic MATa wild-
type or cin8D strains with the indicated
plasmids were grown on plates containing
either 0.1% DMSO alone or with 120 ?M
cincreasin for 2 days and photographed.
codes and hybridizing them to an oligonucleotide
microarray [25–27]. We identified heterozygous dele-
tions sensitive to cincreasin by treating the collection
with the inhibitor and measuring the growth rates over
16 generations relative to those of a DMSO-treated
control (see Supplemental Experimental Procedures for
details). No statistically significant sensitive strains
were identified at 100 ?M cincreasin. At 200 ?M and
400 ?M, 53 and 106 sensitive strains were identified.
All 53 mutants sensitive at 200 ?M were also sensitive
at 400 ?M; 52 of these 53 strains were sensitive when
individually retested on plates (data not shown). We
found that the strain heterozygous for MPS1 was
among the most sensitive in the genome (Table S2;
ranked second). Moreover, the MPS1 heterozygote was
the most sensitive of the known checkpoint strains
(Figures S2A and S2B). Because cincreasin is similar
to indolinones, a known class of kinase inhibitors, and
MPS1 has mitotic- and spindle-checkpoint functions,
we hypothesized that MPS1 could be a target of
We confirmed that MPS1/mps1D heterozygotes are
sensitive to cincreasin by comparing the growth rate of
MPS1/mps1D cells with wild-type yeast in a competi-
tive-growth assay. Because another protein kinase,
Ipl1, is also required for the tension-sensing branch of
the spindle checkpoint, we included this strain as a
control. Equal numbers of log-phase wild-type and mu-
tant cells were mixed, and compound or DMSO was
added to the medium. After 20 generations of growth
in the presence of cincreasin, the MPS1/mps1D hetero-
zygote was 50-fold less abundant in the culture (Figure
4A). In contrast, the IPL1/ipl1D heterozygote was only
slightly sensitive to 200 ?M cincreasin (Figure 4B). We
also found that overexpression of MPS1 on a 2 micron
vector decreases the sensitivity of cin8D cells to
cincreasin and partially restores the ability of cells with
short linear chromosomes to arrest at the spindle
checkpoint in the presence of cincreasin (data not
shown). Taken together, these data suggest that Mps1
is an important target of cincreasin.
Mps1 as a Target of Cincreasin
To confirm that Mps1p is a target of cincreasin, we per-
formed an in vitro kinase assay on purified Mps1p.
Mps1p kinase activity is inhibited in vitro by cincreasin
with an IC50of approximately 700 ?M, with complete
inhibition at 1 mM (Figures 4C and 4D; Figure S4). A
separate test of Ipl1 failed to show inhibition of kinase
activity in vitro (S. Biggins and S. Tatsutani, personal
communication). Cincreasin completely inhibits growth
of wild-type cells at a concentration of 1 mM (Figure
S2), although given the compound’s simple chemical
structure, it is likely to inhibit several targets at this con-
centration. Lower concentrations of cincreasin (200
?M), which affect chromosome segregation and the re-
sponse of the spindle checkpoint to chromosomes that
are not under tension, do not affect viability or doubling
time significantly (data not shown). The modest kinase
inhibition seen in vitro is consistent with the idea that
cincreasin only partially inhibits Mps1 in vivo. Mps1
plays an essential role in duplicating the spindle-pole
body, the yeast equivalent of the centrosome, and be-
cause our screen identified nonlethal compounds that
allow cells to proliferate because they inhibit the spin-
dle checkpoint, these could not have been strong Mps1
inhibitors. Our results suggest that sensing of tension
and attachment by the spindle checkpoint require dif-
ferent levels of Mps1 activity. This suggestion agrees
with recent observations on quantitatively varying Mps1
activity: (1) more Mps1 activity is necessary for centro-
some duplication than the spindle checkpoint ; (2)
overexpression of Mps1 activates the spindle check-
point ; and (3) mps1-1 is partially defective for ki-
nase activity at 23°C  and is synthetically lethal with
mutations in CIN8  but is not defective in sensing
attachment defects at 23°C . Subtle defects in Mps1
activity could abrogate the tension response while pre-
serving the response to severe attachment defects. Ad-
ditional experiments with more-potent Mps1 inhibitors
or a newly-described analog-sensitive allele of Mps1
 to carefully correlate the activity of Mps1 with the
tension and attachment responses of the checkpoint
should shed light on this issue.
To support our conclusion that Mps1 is a target of
cincreasin, we identified dominant mutants, in the ki-
nase domain of MPS1, that confer resistance to cin-
creasin. Six different alleles of MPS1 render cin8D cells
resistant to cincreasin (Table S3). Three of the six alleles
contained an identical change in a single amino acid at
position 602 (T602S), and we confirmed that this is the
sole amino acid change required for the dominant sup-
pression (Figure 4E). A wild-type MPS1 on the same
centromere (CEN) vector was incapable of suppression.
MPS1-T602S mildly suppresses the cincreasin sensitiv-
ity of bub1D cells (data not shown), and this mutation
does not inhibit MPS1 function because the mutant
protein suppresses the temperature sensitivity of an
mps1-1 mutant (data not shown). Threonine 602 is a
residue whose homolog in cAMP-dependent protein ki-
nase plays an important role in positioning a catalytic
aspartic acid, making it likely that mutating this residue
could affect the strength of cincreasin binding or in-
crease the catalytic activity of Mps1 . Taken to-
gether, our genetic and biochemical data suggest that
Mps1 is an important target of cincreasin and that the
lethality caused by cincreasin in cin8D mutants is due
to a direct effect on the Mps1 protein.
Our findings are consistent with recent findings with
an ATP-analog-sensitive allele of Mps1 and explain the
wide range of deletion mutants that are sensitive to cin-
creasin. Cells completely lacking Mps1 kinase activity
have severe defects in mitotic-spindle formation, sister-
kinetochore positioning at metaphase, and chromo-
some segregation during anaphase, in addition to de-
fects in the spindle checkpoint . How Mps1 might
specifically affect the tension-sensing machinery, in-
cluding potential interactions with Ipl1  and Sgo1
 in the checkpoint response, is not yet known.
Mps1 Kinase Inhibitors in Mammalian Cells
We tested the effect of cincreasin on mammalian cells.
We were unable to inhibit the spindle-checkpoint re-
sponse to microtubule depolymerization by nocodazole
or inhibition of the kinesin Eg5 by monastrol in mamma-
lian cells (R.K.D., unpublished data). Nevertheless, the
conserved roles of Mps1 in mitosis and meiosis 
suggest that a more potent and specific inhibitor of
Mps1 would be a useful tool to probe cell division in
mammalian cells. Many tumors display chromosomal
instability, in which they gain or lose chromosomes and
become aneuploid. For most tumors, the molecular ba-
sis of this instability is unknown. Some tumors are de-
fective for the spindle checkpoint and carry mutations
in hBUB1, whereas many others have a functional spin-
dle checkpoint . We suggest that both spindle-
checkpoint-defective tumors and tumors defective for
other aspects of chromosome segregation may be
highly sensitive to inhibition of Mps1. Moreover, Mps1
kinase inhibitors may add a synergistic toxic effect to
current chemotherapeutics that target the mitotic spin-
dle or to newer drugs in development, including Aurora
kinase inhibitors  and Eg5 kinesin inhibitors ,
that target other proteins required for mitosis.
Supplemental Data including Supplemental Experimental Pro-
cedures, four Supplemental Figures, and four Supplemental Tables
are available with this article online at http://www.current-biology.
We thank Christina Cuomo for discussion and help with the TAG3
microarrays and statistical analysis, Marion Shonn Dorer for com-
ments on the manuscript and experimental advice, Bodo Stern,
Alex Szidon, Scott Schulyer, Vahan Indjeian, and members of the
Murray lab for experimental advice and discussion, and Mark
Winey for reagents and helpful discussion. We thank Sue Biggins
and Sean Tatsutani for sharing their unpublished results. This work
was supported by funding from the National Institutes of Health
grants GM043987 (A.W.M.), GM62566 (Institute of Chemistry and
Cell Biology [ICCB]/A.W.M./T.J.M.), and CA96470 (W.H.W./A.W.M.).
R.K.D. was supported by a Howard Hughes Medical Institute post-
doctoral grant for physician-scientists.
Received: February 3, 2005
Revised: April 28, 2005
Accepted: May 3, 2005
Published: June 7, 2005
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