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.