NDUFB9 is an accessory subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (complex I). Human NADH dehydrogenase (ubiquinone) 1beta-subcomplex, 9 (NDUFB9) is a nuclear-encoded mitochondrial protein with the respiratory electron transport chain. It has been physically mapped to a 1-Mb deletion at chromosome 8q13 which also contains the gene for branchio-oto-renal (BOR) syndrome. BOR syndrome is characterized by branchial and renal abnormalities with hearing impairment. Since several hereditary deafness disorders have been associated with mitochondrial mutations, NDUFB9 was considered a candidate gene for BOR syndrome. NDUFB9 is down-regulated in highly metastatic breast cancer cells. Mutant NDUFB9 is a new cause of complex I deficiency.