Uterine leiomyoma is the most common tumor of smooth muscle cell origin and is often associated with the recurrent balanced translocation t(12;14)(q13-15;q24). As an initial step toward finding the gene or genes that are interrupted by the translocation breakpoint, a somatic cell hybrid carrying the derivative 14 as the single t(12;14) translocated chromosome was constructed from a leiomyoma cell line with this translocation. Sequence tagged sites (STS) whose locations on the genetic map of chromosome 14 were known were used to map the breakpoint in the translocated chromosomes. The results of this analysis place the translocation breakpoint on the long arm of chromosome 14 between the proximal marker SPTB and the distal marker D14S77, narrowing the chromosomal translocation breakpoint to a region of approximately 7 cM. The identification of flanking markers on chromosome 14 lays the foundation for efforts to clone the breakpoint and to identify the genes involved in the formation of leiomyoma.