The heavy chain locus is located in the centromeric portion of chr13q (483 Mbp). (A) Gene density plot of chr13q where the IGH locus is encoded (box). Dark blue colors indicate low gene density. (B) Zoom of the whole IGH locus (35.967 – 48.91 Mbp). Non-Ig genes (black) in proximal flank OXA1L gene and distal flank ABHD4, IGHC genes (blue), IGHV genes (red), and IGHD and IGHJ (yellow). (C) Zoomed view (36.9 – 38.09 Mbp) of the IGHJ-IGHC gene cluster. The lower panel shows AID hotspots density per 2.5 Kb along the IGHJ-IGHC cluster to map Switch regions. (D) Close-up of the IGHJ cluster and the IGHM gene. The bottom panel shows the spleen RNA-seq coverage histogram of the IGHJ-IGHM region. (E) Schematic representation of IGH locus in A. mexicanum genome (v6) and the corresponding locus in X. tropicalis. Color code as in A X. tropicalis displays the canonical architecture, in which the V, J and C clusters have the same orientation. Note that in A. mexicanum the JH-CH cluster and 3 IGHV segments appear to be inverted, which seems implausible due to the mechanism of V(D)J recombination. Moreover, IGHV segment orientation in axolotl are intercalated, which is atypical. Interestingly, in axolotl, the TRA/TRD locus (Yellow) is not linked to the IGH locus as in X. tropicalis and is coded across the centromere (black circle) in chr13p: 3.3-7.7 Mbp). Not on scale. The IGHD cluster is not shown for simplicity.