Sanfilippo syndrome type D is a ultra rare syndrome with limited available natural history data. This study is planned to document, through retrospective and prospective data collection, syndrome progression in children and young adults with Sanfilippo syndrome type D. The results from this study may inform future clinical studies in targeted therapies for patients with Sanfilippo syndrome type D and may serve as an external control since there are very few patients with Sanfilippo syndrome type D.

This is a single-center, natural history study of subjects with Sanfilippo syndrome type D. This study will combine a retrospective review of medical records and an ongoing collection of clinical data on an observational basis from participants with Sanfilippo syndrome type D.

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 RARE (Recording Application for Real-World Evidence). The RARE app consists of 9 tasks of Activities of Daily Living and socialization as well as two parent reported questionnaires.

No formal power or sample size calculations will be performed. At least 4 and up to 10 patients living with Sanfilippo syndrome type D will be enrolled in the prospective and retrospective study (group/cohort 1). An additional 5 patients deceased or living will be enrolled for retrospective medical record collection only (group/cohort 2).

The objectives of this study are:

1. To enhance the understanding of the natural history and progression of Sanfilippo syndrome type D.
2. To define and categorize clinical endpoints that may be used in future clinical trials.

Inclusion Criteria:

To be eligible to participate in the study, patients must meet all of the following criteria:

1. Confirmed diagnosis of Sanfilippo syndrome type D disease by all of the following:

   1. Deficiency in alpha-GNS enzyme activity
   2. Has presented with signs/symptoms consistent with Sanfilippo syndrome type D, 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
   3. Genomic DNA analysis demonstrating homozygous or compound heterozygous, pathogenic and/or potentially pathogenic variants in the GNS gene
   4. Accumulated GAG HS in urine

2. Written informed consent from parent or legal guardian and assent from patient, if required

3. Parent/legal guardian willing to accompany the patient to all study visits

4. Ability to comply with protocol requirements, in the opinion of the Investigator

5. Negative urine pregnancy test at screening (nonsterile females of childbearing potential only) -

Exclusion Criteria:

Patients who meet any of the following criteria will not be eligible to participate in the study:

1. Have received an investigational drug within 30 days prior to the Baseline Visit
2. 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
3. The presence of significant non-MPS IIID-related CNS impairment or behavioral disturbances that would confound the scientific rigor or interpretation of results of the study -

Patients with a documented diagnosis of Sanfilippo syndrome type D and who are currently untreated with investigational products (drugs/device) for this disease

The data collected as part of the observational study may be analyzed and published. Phoenix Nest recognizes the importance of communicating medical study data and therefore encourages the publication of these data in reputable scientific journals and at seminars or conferences. The details of the processes of producing and reviewing reports, manuscripts, and presentations based on the data from this study will be described in the Clinical Trial Agreement between Phoenix Nest and the Investigator/institution. Consideration for authorship of all publications will be based on compliance with the Uniform Requirements for Manuscripts Submitted to Biomedical Journals ("Uniform Requirements") of the International Committee of Medical Journal Editors (http://www.icmje.org/ethical\_1author.html) and Good Publication Practices.