Ferredoxin1 is a small iron-sulfur protein that transfers electrons from NADPH through ferredoxin reductase to a terminal cytochrome P450. This particular oxidation/reduction system is found in steroidogenic tissues, and is involved with the synthesis of bile acid and vitamin D. It participates in the synthesis of thyroid hormones and transfers electrons from adrenodoxin reductase to the cholesterol side chain cleavage cytochrome P450.