A consistent, balanced, reciprocal chromosomal translocation t(2:13) (q35:q14) has been identified in alveolar rhabdomyosarcoma. Somatic cell hybrids have been constructed between rhabdomyosarcoma cell lines carrying the (2:13) translocation and mouse 3T3 cells. One hybrid cell line was shown to have retained the derivative (13:2) chromosome, but segregated the normal chromosome 13 and the derivative (2:13) chromosome. Using available DNA probes from human chromosome 13 we find that the loci for retinoblastoma, esterase D, p7D2, pG24E6.8 and pG14E1.9 lie distally to the 13q14 breakpoint, whereas those for p7F12, pHU10 and pG2E3.1 all lie proximally. Thus we have defined a region of 13q14 of approximately 28mB which contains the breakpoint associated with this rearrangement.