Natural History Study of Participants With Sanfilippo Syndrome Type IIIC

This is a multi-center, natural history study of subjects with Sanfilippo syndrome type C or MPS IIIC. This study will combine a retrospective review of medical records and an ongoing collection of clinical data on an observational basis. Real-world data will be collected at home using a recording device. Participants will be asked to attend clinic visits in person on a yearly basis. In clinic visits will include neurocognitive, developmental, behavioral, biochemical, imaging measures as well as retrospective medical record analysis. At 6-month intervals between clinic visits, participants will be contacted remotely for some safety assessments (e.g., collecting information about concomitant medications, concurrent illnesses, and procedure-related adverse events \[AEs\] and serious AEs \[SAEs\] since last clinic visit). At 6-month intervals, caregivers will complete questionnaires and record home video assessments using a dedicated, compliant study application (app) called C-RARE (Recording Application for Real-World Evidence). The C-RARE app consists of 9 tasks of Activities of Daily Living and socialization as well as three parent reported Sanfilippo specific questionnaires. The objectives of this study are: * To enhance the understanding of the natural history and progression of MPS IIIC syndrome on behavior, cognitive abilities, mobility, vision, hearing, speech, and expressive and receptive language * To define and categorize clinical endpoints that may be used in future clinical trials