NDUFS5 and lactic acidosis: Three of these accessory CI subunits NDUFA8, NDUFB7 [40] and NDUFS5 [41] have previously been identified as CHCHD4 substrates with canonical twin CX9C motifs [8], while a fourth subunit NDUFB10 contains a non-canonical cysteine motif (CX6C-CX11C) (Table 1) and has been shown recently to be mutated in a single case of fatal infantile lactic acidosis and cardiomyopathy in a way that renders it defective in binding to CHCHD4 [29].