Multisystem disorder associated with a missense mutation in the mitochondrial cytochrome b gene

Mitochondrial cytochrome b mutations have been reported to have a homogenou s phenotype of pure exercise intolerance. We describe a novel mutation in t he cytochrome b gene of mitochondrial DNA (AI5579G) associated with a selec tive decrease of muscle complex III activity in a patient who, besides seve re exercise intolerance, also has multisystem manifestations (deafness, men tal retardation, retinitis pigmentosa, cataract, growth retardation, epilep sy). The point mutation is heteroplasmic in muscle (88%) and leukocytes (15 %), and changes a highly conserved tyrosine to cysteine at amino acid posit ion 278.