Natural History Study of Participants With Sanfilippo Syndrome Type IIIC

Inclusion Criteria: Confirmed diagnosis of Sanfilippo syndrome type C disease by all of the following: * Deficiency in heparan-alpha-glucosaminide N-acetyltransferase enzyme activity * Has presented with signs/symptoms consistent with Sanfilippo syndrome type C, or, for individuals who have not presented with signs/symptoms of disease (eg, siblings of known patients), the determination of eligibility will be at the discretion of the Sponsor in conjunction with the site Investigator * Genomic DNA analysis demonstrating homozygous or compound heterozygous, pathogenic and/or potentially pathogenic variants in the HGSNAT gene * Accumulated GAG HS in urine * Written informed consent from parent or legal guardian and assent from patient, if required * Parent/legal guardian willing to accompany the patient to all study visits * Ability to comply with protocol requirements, in the opinion of the Investigator * Negative urine pregnancy test at screening (nonsterile females of childbearing potential only). Functional abilities: * Able to take food or liquid by mouth, able to walk with or without assistance. * Has an age equivalent on the Vineland Adaptive Behavior Scales (VABS) of ≥1 year. Exclusion Criteria: Patients who meet any of the following criteria will not be eligible to participate in the study: * Have received an investigational drug within 30 days prior to the Baseline Visit * Concomitant illness or medical condition or extenuating circumstance that, in the opinion of the Investigator, might compromise the patient's ability to comply with protocol requirements, the patient's well-being or safety, or the interpretability of the patient's clinical data * The presence of significant non-MPS IIIC-related CNS impairment or behavioral disturbances that would confound the scientific rigor or interpretation of results of the study