Inclusion Criteria: * Male and female subjects with diagnosis of PKU which is a condition characterized by PAH deficiency * Ability and willingness to maintain dietary protein intake consistent with baseline intake * Willingness to abstain from hepatotoxic substances post-BMN 307 administration * Willingness and capable per investigator opinion to comply with study procedures and requirements * Willingness to use effective methods of contraception * Plasma Phe levels \> 600 µmol/L Exclusion Criteria: * Subjects with primary BH4 deficiency or other forms of BH4 metabolism deficiency * Clinically significant liver dysfunction or disease * Prior treatment with gene therapy * Any condition that, in the opinion of the investigator or Sponsor, would prevent the subject from fully complying with the requirements of the study * History of malignancy