A Study of the Natural Course of SURF1 Deficiency

Inclusion Criteria: * Informed consent/assent provided by the participant based on participant's cognitive ability as determined by Principal Investigator (PI), and/or participant's parent(s) or legally authorized representative(s). * Participant is \< 18 years of age at time of initial informed consent. * Displays one or more clinical features consistent with SURF1 deficiency, including but not limited to, hypotonia, motor delays, motor regression, failure to thrive, language delays, and/or language regression. * Genetic diagnosis of SURF1 pathogenic or likely pathogenic mutation(s), either compound heterozygous or homozygous mutations. If variants are of uncertain significance (VUS), verify documentation of cytochrome c oxidase (COX) activity deficiency. * Ability to travel to the study site and adhere to study-related follow-up examinations and/or procedures and provide access to participant's medical records. Exclusion Criteria: * Any known genetic abnormality (other than SURF1 deficiency), including but not limited to a chromosomal aberration or molecularly known or clinically suspected progressive neurometabolic disorder or dementia, that confounds the clinical phenotype. * The presence of significant non-SURF1-related central nervous system (CNS) impairment/behavioral disturbances that would confound the scientific rigor or interpretation of results of the study or a known history of perinatal asphyxia, kernicterus, carbon monoxide or methanol intoxication. * Current participation in a therapeutic study or participation in a therapeutic study within 30 days prior to enrollment in the present study. * Prior or current treatment with gene or stem cell therapy. * Any condition that, in the opinion of the Site Investigator, could put the participant at undue risk and/or would ultimately prevent the completion of study procedures.