COX18 encodes a cytochrome c oxidase (COX)-assembly protein. The S. cerevisiae Cox18 protein catalyzes the insertion of the Cox2 (MTCO2; MIM 516040) C-terminal tail into the mitochondrial inner membrane, an intermediate step in the assembly of complex IV of the mitochondrial respiratory chain. Required for the insertion of integral membrane proteins into the mitochondrial inner membrane. Essential for the activity and assembly of cytochrome c oxidase. Plays a central role in the translocation and export of the C-terminal part of the COX2 protein into the mitochondrial intermembrane space.