Mitochondrial respiratory chain complex III (ubiquinol-cytochrome c reductase) consists of 11 subunits, only one (cytochrome b) being encoded by the mitochondrial DNA. Disorders of complex III are comparatively rare but are nevertheless present as a clinically heterogeneous group of diseases. To date, no mutation in any of the nuclear-encoded subunits has been described. We report here a deletion in the nuclear gene UQCRB encoding the human ubiquinone-binding protein of complex III (QP-C subunit or subunit VII) in a consanguineous family with an isolated complex III defect. In the proband, a homozygous 4-bp deletion was identified at nucleotides 338-341 of the cDNA predicting both a change in the last seven amino acids and an addition of a stretch of 14 amino acids at the C-terminal end of the protein. Both parents were found to be heterozygous for the deletion, which was absent from 55 controls. Low temperature (-196 degrees C) spectral studies performed on isolated mitochondria from cultured skin fibroblast of the proband showed a decreased cytochrome b content suggestive of a role for the QP-C subunit in the assembly or maintenance of complex III structure.