['Familial hypertrophic cardiomyopathy (FHCM) is an autosomal dominant disease with protean clinical manifestations, ranging from asymptomatic to that of severe heart failure or sudden death. There is no known parameter in individuals with hypertrophic cardiomyopathy (HCM) that predicts a specific clinical event. This is particularly troublesome for premature sudden death that frequently occurs in young athletes without prior symptoms. Recent identification of mutations in the beta myosin heavy chain (beta MHC) gene that co-segregate with the inheritance of the disease provides an opportunity to determine whether certain mutations are more likely to induce a particular clinical event. In this study we analyzed the genotype and phenotype of individuals from two unrelated families with HCM in which the affected individuals have the same missense mutation in exon 13 (G1208A) of the coding sequence for beta MHC.', 'Individuals from two unrelated families with the diagnosis of FHCM were screened by history, physical examination, electrocardiography, and two dimensional echocardiography. After extraction of DNA from the blood of these individuals, the exon 13 of the beta MHC gene was amplified by polymerase chain reaction (PCR), and the PCR product was digested with Ddel restriction endonuclease. The digestion products were separated by gel electrophoresis and identified by ethidium bromide staining.', 'We studied 54 individuals from the two families, 21 were affected with HCM of which eleven died prematurely, eight from sudden cardiac death. While most of the nine affected individuals studied had septal hypertrophy, three had concentric hypertrophy and six, left ventricular outflow tract obstruction. Onset of symptoms was in the second decade of life. Electrophoretic separation of the digested DNA (exon 13) from unaffected individuals provided two fragments of 84 and 70 bp in size, as expected. In contrast, DNA from individuals affected with HCM showed four fragments of 84 bp, 70 bp, 52 bp and 32 bp indicating they inherited the mutation. In only one 10 year old male was the mutation present without evidence of HCM which gives an overall penetrance of 86%.'] Is a missense mutation in the beta myosin heavy chain gene a predictor of premature sudden death in patients with hypertrophic cardiomyopathy?