AbstractA detailed cytogenetic map was constructed around the chromosomal breakpoint of t(8;13) observed in a patient with multiple exostoses. The order of seven loci defined by cosmid clones mapped to 8q23 was determined by means of two‐color fluorescence in situ hybridization (FISH) on elongated prophase chromosomes, and localizations of these markers relative to the breakpoint were examined. The results indicated that loci defined by cC18‐553 and cC18‐1512 flank the breakpoint. By pulsed‐field gel electrophoresis of DNA digested with BssH11 and Southern hybridization with cC18‐1512, DNA from the patient showed a band which was not observed in DNA isolated from either parent. As the normal size of this BssH11 fragment is 600 kb, the chromosomal breakpoint probably lies less than 600 kb away from cC18‐1512.Genes Chrom Cancer 9:57‐61 (1994).© 1994 W