RFLP analysis in Duchenne/Becker muscular dystrophy (D/BMD) has been limited by the lack of informative marker loci at the 3' end of the dystrophin gene. Recently a CACA repeat polymorphism was described in the 3' untranslated end of the dystrophin gene which we have found helpful in genotype assignments of D/BMD families when an RFLP approach is required. The CACA repeat marker has 2 common alleles (1 and 2) that are easily visualized by a nonradioactive PCR method followed by polyacrylamide gel electrophoresis. We present 2 families which demonstrate the use of this polymorphism. Since 35-50% of females are heterozygous, this locus is a useful marker in RFLP analysis of D/BMD families.