Observational Study of Adults and Adolescents With Erythropoietic Protoporphyria (EPP) and X-linked Porphyria (XLP)

Inclusion Criteria: 1. Aged 12 years and older. 2. Confirmed diagnosis of EPP or XLP as follows: 1. Genetic confirmation of EPP (FECH mutation with reduced ferrochelatase activity) OR 2. Genetic confirmation of XLP (ALAS2 gain-of-function mutation) OR 3. Elevated free PPIX levels in RBCs consistent with EPP/XLP, supported by clinical and/or family history. 3. Currently has symptoms of EPP/XLP. 4. History of consistent, non-painful prodrome within approximately 45 minutes of sunlight exposure and prior to phototoxic attacks, as self-reported by the participant. 5. Willing and able to wear a light dosimetry device during the study. 6. Willing and able to complete a daily diary of EPP/XLP symptoms. 7. Willing and able to keep skin sites chosen for provocative sunlight exposure testing covered by opaque material when outside or exposed to potentially triggering light beginning 2 days prior to testing. 8. Willing and able to provide informed consent and/or assent for the study. 9. Study participants and LAR (as appropriate) are willing and able to comply with study visits and study procedures. Exclusion Criteria: 1. Diagnosis of another porphyria or another photodermatosis that may confound the characterization of EPP/XLP. 2. Has a clinically significant disease or condition that, at the discretion of the Investigator or Sponsor, would interfere with the evaluation of EPP/XLP or study participation, or would make study participation not in the best interest of the participant. 3. Is taking, or has taken within 60 days of Day 1, any medication, vitamin, or supplement that alters sensitivity to light exposure (eg, afamelanotide, melanotan, beta carotene, dersimelagon, or bitopertin). 4. Concurrent or anticipated participation in an interventional clinical trial during the study period.