Accessory subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), that is believed not to be involved in catalysis. Required for proper complex I assembly (PubMed:28671271). Complex I functions in the transfer of electrons from NADH to the respiratory chain. The immediate electron acceptor for the enzyme is believed to be ubiquinone. {ECO:0000269|PubMed:22114105, ECO:0000269|PubMed:27626371, ECO:0000269|PubMed:28671271}. This is the function of NDUFA9 (NADH:ubiquinone oxidoreductase subunit A9, Ensembl gene identifier ENSG00000139180).