The Single Large-Scale mtDNA Deletion Sydrome: Natural History Study (PS-NHS) aims to collect data on standardized clinical outcomes, store data on the Champ Foundation Registry (CFR) and make this data available to researchers, clinicians, and industry partners who are studying SLSMDS to answer questions regarding the disease, including its causes, potential treatments, and other topics. A secondary aim is to analyze the data to understand research questions relating to the natural history of SLSMDS.

This study is a prospective, observational, and longitudinal study intended to track the course of Pearson syndrome and single large scale mitochondrial DNA deletion syndromes (SLSMDS) to identify demographic, genetic, environmental, and other variables that correlate with the diseases development and outcomes. If available, retrospective clinical data may be accessed and used in analyses as well.

The PS-NHS will be conducted at two Center of Excellence sites: the Cleveland Clinic and Children's Hospital of Philadelphia (CHOP).

All PS-NHS data will be entered and stored on the CFR. The CFR exists entirely online.

Inclusion Criteria:

• Have an active account on the Champ Foundation Registry (CFR) or be willing to create an account on the CFR.
• Must have a genetic diagnosis of a single large-scale mitochondrial DNA deletion and must upload their genetic report to the CFR.
• Have a clinical diagnosis or history of Pearson syndrome OR have symptom onset prior to five years of age and a genetic diagnosis of a single large-scale mitochondrial DNA deletion OR in the opinion of the principal investigator the participant is suitable for participating in this study based on clinical presentation.

Participants may be of any age or gender, and originate from any country.

All patients with confirmed Pearson Syndrome who satisfy the inclusion/exclusion criteria will be offered enrollment into this study.

The data obtained during the study may be available on reasonable request.