F amilial hypertrophic cardiomyopathy is a disease generally believed to be caused by mutations in sarcomeric proteins. In a family with hypertrophic cardiomyopathy linked to polymorphic markers on chromosome 11, we found a new mutation of a splice donor site of the cardiac myosin-binding protein-C gene. This mutation causes the skipping of the associated exon in mRNA from lymphocytes and myocardium. Skipping of the exon with a consecutive reading frame shift leads to premature termination of translation and is thus expected to produce a truncated cardiac myosin-binding protein-C with loss of the myosin- and titin-binding COOH terminus. However, Western blot analysis of endomyocardial biopsies from histologically affected left ventricular myocardium failed to show the expected truncated protein. These data show for the first time that a splice donor site mutation in the myosin-binding protein-C gene is transcribed to cardiac mRNA. Truncated cardiac myosin-binding protein-C does not act as a "poison polypeptide," since it seems not to be incorporated into the sarcomere in significant amounts. The absence of mutant protein and of significantly reduced amounts of wild-type protein in the presence of the mutated mRNA argues against the "poison protein" and the "null allele" hypotheses and suggests yet unknown mechanisms relevant to the genesis of chromosome-11- associated familial hypertrophic cardiomyopathy.