Dystrophin-related protein (DRP or 'utrophin') is localized in normal adult muscle primarily at the neuromuscular junction. In the absence of dystrophin in Duchenne muscular dystrophy (DMD) patients, DRP is also present in the sarcolemma. DRP is expressed in fetal and regenerating muscle and may play a similar role to dystrophin in early development, although it remains to be determined whether DRP can functionally replace dystrophin in adult tissue. Previously we described a 3.5-kilobase complementary DNA clone that exhibits 80 per cent homology to the C-terminal domain of dystrophin. This sequence identifies a 13-kilobase transcript that maps to human chromosome 6 (refs 2, 11). Antibodies raised against the gene product identify a polypeptide with a relative molecular mass of about 400K in all tissues examined. To investigate the relationship between DRP and dystrophin in more detail, we have cloned and sequenced the whole DRP cDNA. Homology between DRP and dystrophin extends over their entire length, suggesting that they derive from a common ancestral gene. Comparative analysis of primary sequences highlights regions of functional importance, including those that may mediate the localization of DRP and dystrophin in the muscle cell.