COX IV or COX4I1 is a protein that is located in the inner mitochondrial membrane and is an isoform of the nuclear-encoded subunit IV of cytochrome c oxidase (complex IV), the terminal oxidase in mitochondrial electron transport. Complex IV is a multi-subunit enzyme complex that couples the transfer of electrons from cytochrome c to molecular oxygen and contributes to a proton electrochemical gradient across the inner mitochondrial membrane. The expression of COX4I1, along with COX4I2, may be regulated by oxygen levels, with reduced levels of oxygen leading to increased COX4I2 expression and COX4I1 degradation. This suggests a role for COX4I1 in the optimization of the electron transfer chain under different conditions.