Cf56a is a 0.9 kb EcoRI fragment of dystrophin cDNA in pUC13. Cf56a is identical to Kunkel's cDNA probe 8. Constant bands of 14.4, 11.0, 8.1, 6.2 and 1.3 kb correspond to exons I, N, L, N and K respectively. The polymorphic band is exon J (exon 48, 1.2+3.9 kb HindIII bands). This exon is deleted in 25% of all Duchenne/Becker dystrophy boys. Therefore this RFLP is useful for determining carrier status of at-risk females by showing heterozygosity or apparent non-maternity.