 The protein remains functional in areas of skin that are coldersuch as its legs, ears, tail and faceso the cat has dark hair at its extremities.Environment plays a major role in effects of the human genetic disease phenylketonuria.The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine, causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive intellectual disability and seizures.