NADPH-Cytochrome P450 Reductase (P450R) is an essential component of the cytochrome P450 monooxygenase system of eukaryotic cells. P450R is anchored in the endoplasmic reticulum membrane with its catalytic domain residing in the cytosol. P450R is a flavoprotein, containing one molecule each of FMN and FAD, which are essential for the transfer of electrons from NADPH to the cytochromes P450. This reduction is necessary for cytochromes P450 to perform each cycle of oxidation. P450R is also capable of transferring electrons to cytochrome b5, heme oxygenase, the fatty acid elongation system, and other proteins. Mutations of P450R can result in disordered steroidogenesis and Antley-Bixler syndrome.