The purpose of this study is to collect information about the effects of EPP/XLP in adults and adolescents. This is an observational study in which participants will not receive any treatment. Study details include:

• The study duration will be up to 6 months for each participant.
• After Screening, participants will have a Baseline (Day 1) visit and return for visits every 4 weeks through 24 weeks.
• Assessments to evaluate EPP/XLP will include clinical laboratory tests, physical exams, vital signs measurement, blood draws for PPIX concentration, reporting of EPP/XLP-related symptoms (including completion of a daily diary), light dosimetry.

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.

The study design and key inclusion criteria are focused on EPP/XLP and are inclusive of all potential participants, including those from underrepresented racial and ethnic populations. The sites and countries selected have diverse populations and the potential to enroll diverse participants.

What may be shared:

De-identified individual participant data (no direct identifiers)

Who can access it:

Qualified researchers with a scientifically sound research proposal

Conditions for access:

Submission and approval of a research proposal Execution of a data sharing agreement (DSA) Compliance with privacy regulations (e.g., GDPR, HIPAA as applicable)

How it will be shared:

Through a secure data access process managed by the sponsor or designated data-sharing platform

Timing:

Typically after study completion and/or publication of primary results

Purpose:

To support further research on EPP and XLP while protecting participant confidentiality