Respiratory complex I (NADH:ubiquinone oxidoreductase EC 1.6.5.3) is a large mitochondrial inner membrane enzyme consisting of 45 subunits and 8 iron-sulfur (Fe/S) clusters. NUBPL, or IND1, is an Fe/ S protein that plays a critical role in the assembly of respiratory complex I, likely by transferring Fe/S into the Fe/S-containing complex I subunits (Sheftel et al., 2009 [PubMed 19752196]).