We report the cDNA cloning, chromosomal localization, and a mutation in the human nuclear gene encoding the 18-kD (AQDQ) subunit of the mitochondrial respiratory chain complex I. The cDNA has an open reading frame of 175 amino acids and codes for a protein with a molecular mass of 23.2 kD. Its gene was mapped to chromosome 5. A homozygous 5-bp duplication, destroying a consensus phosphorylation site, in the 18-kD cDNA was found in a complex I-deficient patient. The patient showed normal muscle morphology and a remarkably nonspecific fatal progressive phenotype without increased lactate concentrations in body fluids. The child's parents were heterozygous for the mutation. In 19 other complex I-deficient patients, no mutations were found in the 18-kD gene.