[show abstract][hide abstract] ABSTRACT: Tyrosine phosphorylation plays a key role in signal transduction and the regulation of a broad set of physiological processes characteristic of multicellular organisms. Hematopoietic protein tyrosine phosphate (HePTP) is a tyrosine phosphatase expressed in hematopoietic cells with specificity for the dephosphorylation of Erk and p38 MAP kinases (MAPKs). It has been found that HePTP is often dysregulated in the preleukemic disorder myelodysplastic syndrome, as well as in acute myelogeneous leukemia. The identified probe ML119 (CID-1357397) selectively inhibits HePTP activity. Since this probe scaffold was discovered through its activity in the HePTP assays with small molecule substrates, it is proposed that this inhibition occurs through direct targeting of the HePTP active site. The probe inhibits through fast equilibrium as judged by the lack of apparent effect when pre-incubated with phosphatase proteins. Thus, small molecule inhibitors of HePTP would be useful as molecular probes for studying the mechanism of signal transduction and MAP kinase regulation. In addition, these probes may have therapeutic potential for the treatment of hematopoietic malignancies, such as acute myeloid leukemia, where HePTP has been reported to be overexpressed.