This long-term observational study is designed to follow subjects who, during another Clinical Study, received gene therapy treatment used to treat their Homozygous Familial Hypercholesterolemia (HoFH) disease. This study is intended to follow those subjects for up to 5 years since they received treatment to look for any long-term safety concerns. There is no investigational drug or therapy provided as part of this study.

Homozygous Familial Hypercholesterolemia (HoFH) is a rare genetic metabolic disorder characterized by absent or severely reduced capacity to catabolize circulating LDL particles by the hepatic LDL receptor. As a consequence, HoFH subjects present abnormal total plasma cholesterol (LDL-C) levels, resulting in severe atherosclerosis often leading to early onset of cardiovascular disease. Early initiation of aggressive treatment for these patients is therefore essential. Unfortunately, despite existing therapies, treated LDL-C levels could remain well above acceptable levels. Thus, the functional replacement of the defective LDLR via AAV-based liver-directed gene therapy, RGX-501, may be a viable approach to treat this disease and improve response to current lipid-lowering treatments.

This is a prospective, observational study to evaluate the long-term safety and efficacy after a single administration of RGX-501. Eligible participants are those who previously have enrolled in a clinical study and received a single intravenous infusion of RGX-501.

Inclusion Criteria:

• To be eligible to participate in this study, a participant must have previously received RGX-501 in a separate parent trial, and the participant or participant's legal guardian(s) is/(are) willing and able to provide written, signed informed consent after the nature of the study has been explained, prior to any research-related procedures.

Exclusion Criteria:

• None

Participants who have previously received RGX-501 in a separate parent trial