CMC2 (C-X9-C motif containing 2) is a conserved mitochondrial protein essential for cytochrome c oxidase biogenesis and copper metabolism. CMC2 localizes to the mitochondrial inner membrane facing the intermembrane space and contains the characteristic twin Cx9C motif found in metallochaperone-like proteins 1. The protein is essential for cytochrome c oxidase activity, as deletion of CMC2 in yeast results in undetectable cytochrome c oxidase activity and cellular respiration 1. CMC2 functions cooperatively with its homologue CMC1 in cytochrome c oxidase biogenesis, though they have non-overlapping functions - CMC1 overexpression cannot rescue cmc2 mutants and vice versa 1. The protein physically interacts with CMC1, and absence of CMC2 leads to a 5-fold increase in Cmc1 accumulation in mitochondrial membranes 1. CMC2 also affects mitochondrial CuZn-superoxide dismutase (Sod1) activity, with cmc2 mutants showing 2-fold increased Sod1 activity 1. The function is evolutionarily conserved, as human CMC2 localizes to mitochondria and its knockdown produces cytochrome c oxidase deficiency in C. elegans 1. These findings establish CMC2 as a critical component of mitochondrial copper delivery systems essential for respiratory complex assembly.