We have characterized deletions of the dystrophin gene in patients suffering from relatively mild muscular dystrophy. Our data show that most of the Becker muscular dystrophy (BMD) patients have intragenic deletions which leave the protein reading frame in phase. Remarkably, large deletions of the region corresponding to the central triple helical repeats in the protein can result in an exceptionally mild phenotype. Three brothers suffering from BMD, glycerol kinase deficiency, and adrenal hypoplasia possess a deletion at the 3' end of the gene. They also display developmental delay. Thus the 3' processing of the gene must be necessary for the correct function of the dystrophin molecule.