Biogenesis of cytochromec oxidase (COX) is a highly complex process involving 30 chaperones in eukaryotes; those required for the incorpora-tion of the copper and heme cofactors are also conserved in bacteria. Surf1, associated with hemeainsertion and with Leigh syndrome if defec-tive in humans, is present as two homologs in the soil bacterium Paracoc-cus denitrificans, Surf1c and Surf1q.