Cytochrome P450 oxidoreductase deficiency is a rare disease and inborn error of metabolism caused by deficiency of cytochrome P450 oxidoreductase. POR is a 2-flavin protein that is responsible for the transfer of electrons from NADPH to all 50 microsomal cytochrome P450 enzymes. This includes the steroidogenic enzymes CYP17A1, CYP19A1, and CYP21A2 ; CYP26B1 ; and the hepatic drug-metabolizing CYP450 enzymes, among many other CYP450 enzymes. Symptoms of severe forms of PORD include ambiguous genitalia in males and females, congenital adrenal hyperplasia, cortisol deficiency, and Antley–Bixler skeletal malformation syndrome, while symptoms of mild forms include polycystic ovary syndrome in women and hypogonadism in men. Maternal virilization also occurs in severe forms, due to aromatase deficiency in the placenta. The ABS component of severe forms of PORD is probably caused by CYP26B1 deficiency, which results in retinoic acid excess and defects during skeletal embryogenesis. All forms of PORD in humans are likely partial, as POR knockout in mice results in death during prenatal development.