Overview of IGH translocations. (A) Schematic of the IGH locus. The human IGH locus is located adjacent to the telomere of chromosome 14q. The locus is composed of dispersed variable (VH, denoted by green bars), diversity (DH, red), and joining (JH, blue) region segments that undergo somatic recombination in B-cell precursors to produce a functional VDJ unit. These are expressed with constant region gene segments (CH, yellow) with transcription being driven by enhancers (denoted by pink diamonds). (B) A diagram of IGH translocation in BCP-ALL. As a result of an error of D-J or V-DJ recombination, a partner gene from another chromosomal region becomes adjacent to J or DJ region. Expression of the translocated gene is driven by the transcriptional enhancers within the IGH locus. Usually, the entire open reading frame of the translocated gene is maintained without mutation. (C) Schema showing breakpoints of CRLF2/IGH translocation and PAR1 interstitial deletion. Red arrows indicate the positions of break points of the cloned IGH translocations (falling in the region immediately 5′ of CRLF2), whereas the portion consisting of lines and frames in red represents the deletion between P2RY8 and CRLF2 resulting in fusion of the 2 genes.