A cDNA clone encoding human NADH:ubiquinone oxidoreductase (complex I of mitochondrial respiratory chain) MLRQ subunit was isolated from human fetal liver cDNA library. The clone contained an open reading frame of 246 by which predicted a protein comprising 81 amino acids with a calculated molecular weight of 9,370 Da. The deduced amino acid sequence exhibited 95% homology (88% identity and 7% favored substitution) to that of bovine MLRQ subunit. Northern analysis revealed that the cDNA clone hybridized with a 0.7 kb mRNA species which was present in all tissues examined. The expression level of the 0.7 kb mRNA in heart, skeletal muscle, and brain was higher than in other organs. Human MLRQ cDNA could cross-hybridize with the genomic DNAs from various species.