UROD and Tangier disease: PCT (type I) is caused by extrinsic factors contributing to liver disease, such as alcohol consumption, tobacco use, estrogen therapy, hepatitis C virus and human immunodeficiency virus infection, iron overload, HH, and exposure to polychlorinated hydrocarbons, all of which result in decreased UROD activity restricted to the liver.1, 2, 3 In contrast, type II PCT is autosomal dominant with incomplete penetrance in which reduced levels of UROD activity of up to 50% can be found in the liver, red blood cells, and fibroblasts.2