The dystrophin-based membrane cytoskeleton of muscle fibers has emerged as a critical multiprotein complex which seems to impart structural integrity on the muscle fiber plasma membrane. Deficiency of dystrophin causes the most common types of muscular dystrophy, Duchenne and Becker muscular dystrophies. Muscular dystrophy patients showing normal dystrophin protein and gene analysis are generally isolated cases with a presumed autosomal recessive inheritance pattern (limb-girdle muscular dystrophy). Recently, linkage and candidate gene analyses have shown that some cases of limb-girdle muscular dystrophy can be caused by deficiency of other components of the dystrophin membrane cytoskeleton. The most recently identified component, δ-sarcoglycan deficiency occurred in other world populations, to identify the range of mutations and clinical phenotypes, and to test for the biochemical consequences of δ-sarcoglycan gene mutations, we studied Duchenne-like and limb-girdle muscular dystrophy patients who we had previously shown not to exhibit gene mutations of dystrophin, α-, β-, or γ-sarcoglycan for δ-sarcoglycan mutations (n = 54). We identified two American patients with novel nonsense mutations of δ-sarcoglycan (W30X, R165X). One was apparently homozygous, and we show likely consanguinity through homozygosity for 13 microsatellite loci covering a 38 cM region of chromosome 5. The second was heterozygous. Both were girls who showed clinical symptoms consistent with Duchenne muscular dystrophy in males. Our data shows that δ-sarcoglycan deficiency occurs in other world populations, and that most or all patients show a deficiency of the entire sarcoglycan complex, adding support to the hypothesis that these proteins function as a tetrameric unit.