In 2004, Rieger et al discovered cytochrome CYP4Z2P, a pseudogene of cytochrome P450 family 4 subfamily Z member 1 (CYP4Z1) gene, and showed that there was an up-regulation of both CYP4Z2P and CYP4Z1 in breast cancer.135. Here, CYP4Z1 is linked to breast cancer.