We studied twin sisters, in their sixth decade, who were obligate carriers of Duchenne dystrophy. One had a slowly progressing limb-girdle myopathy since her mid-20s. The other sister showed no evidence of neuromuscular disease by history or on physical examination but had high serum CK values and degeneration and regeneration of fibers in a muscle biopsy. Otherwise, they were phenotypically identical, karyotypically normal females with cytogenetically normal X-chromosomes. Based on red cell and HLA loci antigen determinations, there was a 99.2% probability that they were monozygotic. The mutant gene segregating in the family is probably linked to the Xp21 DNA marker pERT87.
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Chutkow et al. (1987) studied this question.