A Study to Provide Access to Mitapivat for Participants Who Previously Participated in an Agios-Sponsored Mitapivat Study

Inclusion Criteria: * Participating in an Agios-sponsored study of mitapivat and deriving clinical benefit from ongoing mitapivat treatment, in the opinion of the Investigator. * For women of childbearing potential (WOCBP), must be abstinent of sexual activities that may result in pregnancy as part of their usual lifestyle or agree to use 2 forms of contraception, 1 of which must be considered highly effective, from the time of providing informed consent/assent, throughout the study, and for 28 days after the last dose of study drug. The second form of contraception can be an acceptable barrier method. * Written informed consent/assent from the participant (or their legally authorized representative, parent\[s\], or legal guardian) before any study-related procedures are conducted, and participants must be willing to comply with all study procedures for the duration of the study. * Agrees not to participate in other interventional clinical studies, including other studies of mitapivat, during participation in this study. Exclusion Criteria: * Known allergy or other contraindication to mitapivat or its inactive ingredients as defined in the current investigator brochure (IB). * Any medical, hematologic, psychological, or behavioral condition(s) or prior or current therapy that, in the opinion of the Investigator, may confer an unacceptable risk to participating in the study and/or could confound the interpretation of the study data. Also excluded are: * Participants who are institutionalized by regulatory or court order. * Participants with any condition(s) that could create undue influence (including but not limited to incarceration, involuntary psychiatric confinement, and financial or familial affiliation with the Investigator or Sponsor).