A Dose-finding Study to Evaluate mRNA-3210 in Participants With Phenylketonuria

Inclusion Criteria: * Confirmed diagnosis of PKU due to phenylalanine hydroxylase (PAH) deficiency by molecular genetic testing from a central lab. * At least 3 blood phenylalanine levels ≥600 micromole(μmol)/Litre (L) regardless of diet: 2 obtained during the screening period (at least 72 hours apart) and at least one historical value 6 to 24 months prior to start of screening. * Have received documented approval from a study dietitian confirming that participant is willing and able to maintain dietary protein intake consistent with baseline intake during study participation. * If applicable, maintained stable dose of neuropsychiatric medication (that is, for attention deficit hyperactivity disorder (ADHD), depression, anxiety, or other psychiatric disorders) prior to enrollment and willing to maintain stable dose throughout study participation unless, per investigator assessment, a change is clinically indicated. Exclusion Criteria: * Receipt of sapropterin or large-neutral amino acids within 14 days or 5 half-lives (whichever is longer) of the start of screening. * Receipt of pegvaliase within 2 months of start of screening. * For participants previously on pegvaliase: use or planned use of any injectable drugs containing polyethylene glycol (PEG), including medroxyprogesterone injection, within 3 months prior to the start of screening and during study participation with the exception of COVID-19 vaccinations. * Receipt of any investigational drug within 30 days or 5-half-lives (whichever is longer) of screening. * History of hypersensitivity to any component/excipient used in this study. * Any other clinically significant medical condition that, in the Investigator's opinion, could interfere with the interpretation of study results or limit the participant's participation in the study Note: Other protocol-defined inclusion/exclusion criteria apply.