Coproporphyrinogen-III oxidase, mitochondrial precursor, also known as CPOX, is a 454 amino acid mitochondrial enzyme that is localized to the inner membrane space of erythrocytes. It participates in the sixth step of heme biosynthesis by catalyzing the formation of protoporphyrinogen IX from coproporphyrinogen III. Mutations in human CPOX gene predict the clinical outcome of the disease, with either hepatic hereditary coproporphyria or hematological manifestations of erythropoietic harderoporphyria.