Three families, in which several male individuals suffer from a hereditary neuromuscular disease, were examined by analysis of naturally occurring restriction fragment length polymorphisms (RFLPs) and by screening for deletions. Originally, differential diagnosis included spinal muscular atrophy (two families) and limb girdle syndrome (one family) or Becker muscular dystrophy. Since deletions were not detectable, an X-chromosomal segment, carrying DNA markers for the dystrophin gene and its flanking regions was reconstructed; this demonstrated Becker muscular dystrophy is the most probable primary cause of illness in these families. Furthermore, the possible carriership of female members of these families could be determined accurately.