We report the molecular characterization of a reciprocal constitutional translocation t(7;22)(p13;q11.2) carried by three family members who have each developed a hematological malignancy. The chromosome 7 breakpoint was localized to a single BAC clone, RP11-571N3, by sequential fluorescence in situ hybridization analysis of clones selected from the NCBI chromosome 7 map. This was further refined to a 739-bp region by Southern blot analysis of DNA from the two cell lines 1193 and 1194 digested with EcoRI, HindIII, PstI, and PvuII. A 2.8-kb fragment spanning the der(22) breakpoint was amplified by long-range inverse PCR. The sequence of this fragment was used to predict the composition of the der(7) breakpoint, and a 1.3-kb fragment was amplified by use of primers from both chromosomes 7 and 22 based on this prediction. The breakpoint on chromosome 22 is located between the 3rd and 4th V regions of the immunoglobulin lambda (IGL) locus, and the breakpoint on chromosome 7 is located 122 kb proximal to the insulin-like growth factor binding protein (IGFBP) 3 gene. Examination of both reciprocal junctions showed that four bases were lost from chromosome 22, whereas 75 bases were lost from chromosome 7. Small insertions of 46 bases and 13 bases were found at the der(22) and the der(7) junctions, respectively. As a consequence of this event, the entire IGL locus, less the first three Vlambda elements, is translocated to chromosome 7, whereas the three remaining Vlambda elements on the der(22) are juxtaposed with IGFBP3 and IGFBP1.