This is a Phase 1/2, open-label, dose escalation study to evaluate the safety, efficacy and tolerability of BMN 307 in adult PKU subjects with PAH deficiency. Participants will receive a single administration of BMN 307 and will be followed for safety and efficacy.

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