The majority of Duchenne and Becker muscular dystrophy cases are caused by deletions observable in Southern blots with cDNA probes for the gene. When the deletion includes polymorphic probes, they may be used to determine carrier status by deletion segregation analysis: non-inheritance of parental alleles, or heterozygosity. The polymorphic genomic probe P20 is deleted in a large percentage of probands. P20 hybridizes with two constant fragments of 6.7 and 0.8 kb in Taql digests. In a number of probands, only the larger P20 Taql fragment is deleted. This study demonstrates that this fragment corresponds with the polymorphic EcoRV and Mspl fragments of P20. Families in which the upper Taql fragment is deleted may be screened for carrier status using non-inheritance of parental alleles or heterozygosity of P20 in EcoRV or Mspl digests.