Factor Inhibiting Hypoxia Inducible Factor (FIH) has an important role in the response to low oxygen (hypoxia) in animals. FIH adds a hydroxyl (-OH) group onto the master regulator Hypoxia Inducible Factor (HIF), influencing its ability to activate hundreds of genes that mediate the body’s response to hypoxia. It also hydroxylates members of the ankyrin repeat domain protein family, including adding a single hydroxyl group to ASPP2 on asparagine 986.