Two somatic cell hybrids containing the der(12) or the der(X) from a mesothelioma with a translocation t(X;12)(q22;p13) as the only chromosomal change were generated to characterize the region of 12p13 containing the translocation breakpoint. Fluorescence in situ hybridization analysis showed the breakpoint on chromosome 12 to occur between VWF and D12S158. On the linkage map developed by J. Weissenbach et al. (Nature, 1992, 359: 794-801), the breakpoints were located between D12S99 and D12S100 on chromosome 12 and between DXS1106 and DXS1001 on chromosome X. PCR analysis based on genomic sequences, with DNA from both somatic cell hybrids, enabled us to map CACNL1A1, FGF6, D12S370, D12S38OE, D12S381E, and D12S382E distally to the 12p13 breakpoint and to VWF.