RGX-121-5101 is the long-term follow-up study to the RGX-121-101 first in human study where participants received RGX-121, a gene therapy intended to deliver a functional copy of the iduronate-2-sulfatase gene (IDS) to the central nervous system. This study will evaluate the long-term safety and efficacy of RGX-121.

This is a prospective follow-up study to evaluate the long-term safety and efficacy after a single administration of RGX-121. Eligible participants are those who previously have enrolled in clinical study RGX-121-101 and received a single intracisternal (IC) or intracerebroventricular (ICV) infusion of RGX-121. Enrollment of each participant will occur the same day or after the participant has completed the end of study (EOS) visit or early termination visit (ET) from the previous (parent) study. Participants will be followed in this study cumulatively for up to 5 years after RGX-121 administration (inclusive of the parent study) or until RGX-121 is commercially available in the participant's country, whichever occurs first. No treatment will be directed under this observational protocol. The total study duration for each participant may vary depending on when they enroll in the current study following RGX-121 administration in the parent study.

Inclusion Criteria:

• To be eligible, a participant must have previously received RGX-121 in a separate parent trial.
• Participant or participant's legal guardian(s) is/(are) willing and able to provide written, signed informed consent

Exclusion Criteria:

• Patient has not received RGX-121 previously in a separate parent trial.

Up to 12 subjects over the age of 28 months who have received RGX-121 in a previous study.