This is a family of short, approximately 105 amino acid residue, proteins which form part of NADH:ubiquinone oxidoreductase complex I. Complex I is the first multisubunit inner membrane protein complex of the mitochondrial electron transport chain and it transfers two electrons from NADH to ubiquinone. The protein carries four highly conserved cysteine residues but these do not appear to be in a configuration which would favour metal binding so the exact function of the protein is uncertain [1].

1: The human NADH: ubiquinone oxidoreductase NDUFS5 (15 kDa) subunit: cDNA cloning, chromosomal localization, tissue distribution and the absence of mutations in isolated complex I-deficient patients. Loeffen J, Smeets R, Smeitink J, Triepels R, Sengers R, Trijbels F, van den Heuvel L; J Inherit Metab Dis. 1999;22:19-28. PMID:10070614