Human phenylketonuria (PKU) results from phenylalanine hydroxylase (PAH) deficiency, and represents one of the most common and extensively studied single-gene Mendelian disorders in humans. Unfortunately, optimum clinical outcome demands lifelong dietary restriction through adherence to an unpalatable and expensive artificial diet. Challenges in maintaining traditional therapy lead to increasing phenylalanine (Phe) levels in patients as they approach adulthood with an incumbent severe burden of psychosocial and intellectual difficulties. The recent introduction of the new medication Sapropterin for treatment of PKU has improved Phe control and dietary tolerance in some patients, but at enormous cost to patients and insurers for the FDA designated orphan product. Thus, there is an unmet need for novel therapies to correct PKU. PAH is almost exclusively expressed in the liver in humans. The main objective of the current proposal is to determine the feasibility of hepatocyte transplantation to correct the biochemical (and ultimately, clinical) features of PKU.