Expression of three isoforms produced by the endogenous locus was disrupted by the replacement of exon 52 with a neomycin selection cassette. Western blot analysis showed that while both the 116 kD peripheral nerve and the 71 kD non-muscle isoforms were present in mutant mice, the 427 kD dystrophin, 260 kD retinal, and 140 kD isoforms were absent.