Ferredoxin reductase is a ubiquitous flavoenzyme, containing noncovalently bound FAD as a prosthetic group. It plays a role in delivering NADPH or low potential one-electron donors such as ferredoxin and flavodoxin to redox-based metabolisms in plastids, mitochondria and bacteria. In mammals, ferredoxin reductase is loosely associated with the inner mitochondrial membrane and receives electrons from NADPH. These electrons are transferred to ferredoxin which shuttles electrons to cytochrome P450 in the adrenal cortex mitochondrial steroid hydroxylation systems. It serves as the first electron transfer protein in all the mitochondrial P450 systems. Including cholesterol side chain cleavage in all steroidogenic tissues, steroid 11-beta hydroxylation in the adrenal cortex, 25-OH-vitamin D3-24 hydroxylation in the kidney, and sterol C-27 hydroxylation in the liver.