The purpose of this research study is to determine whether partial irradiation of the liver and liver cell transplantation can reduce the need for dietary and medical management or could possibly eliminate the need for a special diet and medications to treat this disease for patients with phenylketonuria (PKU) by normalizing phenylalanine levels in the body. Phenylalanine (Phe) is a substance needed in the body that can only be obtained from the foods people eat. People with PKU cannot get rid of Phe in their body. Large amounts of Phe can cause problems, such as deterioration of mental function. At the present time, liver cell transplants are experimental and have been done in only a limited number of human subjects.