Central core disease (CCD) and nemaline myopathy (NM) are congenital myopathies for which differential diagnosis is often based on the presence either of cores or rods. Missense mutations in the skeletal muscle ryanodine receptor gene (RYR1) have been identified in some families with CCD. Mutations in the alpha-tropomyosin and alpha-actin genes have been associated with most dominant forms of NM. Analysis of the RYR1 cDNA in a French family identified a novel Y4796C mutation that lies in the C-terminal channel-forming domain of the RyR1 protein. This mutation was linked not only to a severe and penetrant form of CCD, but also to the presence of rods in the muscle fibres and to the malignant hyperthermia susceptibility (MHS) phenotype. The Y4796C mutation was introduced into a rabbit RYR1 cDNA and expressed in HEK-293 cells. Expression of the mutant RYR1 cDNA produced channels with increased caffeine sensitivity and a significantly reduced maximal level of Ca(2+) release. Single-cell Ca(2+) analysis showed that the resting cytoplasmic level was increased by 60% in cells expressing the mutant channel. These data support the view that the rate of Ca(2+) leakage is increased in the mutant channel. The resulting chronic elevation in myoplasmic concentration is likely to be responsible for the severe expression of the disease. Haplotyping analysis indicated that the mutation arose as a neomutation in the proband. This first report of a neomutation in the RYR1 gene has strong implications for genetic linkage studies of MHS or CCD, two diseases characterized by a genetic heterogeneity.