Limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy (MM) are autosomal recessive disorders caused by mutations in the dysferlin gene on chromosome 2p13. The authors studied a large Russian family with both LGMD2B and MM. All affected individuals, as well as one preclinical boy with dystrophic changes on muscle biopsy, were found to be homozygous for a novel dysferlin mutation, TG573/574AT (Val67Asp). This finding supports the view that additional factors (e.g., modifier genes) contribute to the phenotypic expression of causative mutations in dysferlinopathies.