A Natural History Study of Angelman Syndrome

Inclusion Criteria: * The participant has a primary clinical diagnosis of Angelman syndrome with documented genetic variation(s) affecting the function of the UBE3A gene within the human 15q11.2-q13.3 locus. Co-occurring conditions (e.g., autism spectrum disorder, cerebral palsy, intellectual disability) are permitted; however, Angelman syndrome must be the primary clinical diagnosis. * The participant is male or female (assigned sex at birth) and aged ≥1 year at the initial study visit. * The participant has a study partner who meets the study partner criteria below. * The participant, if unable to provide informed consent, has an appropriate surrogate who is at least 18 years of age and willing and able to provide informed consent on behalf of the participant in accordance with current International Council for Harmonisation (ICH) guidelines and applicable institutional regulations. Individuals must satisfy the following criteria to be enrolled as study partners: * The study partner is a parent or primary caregiver who is at least 18 years of age. * The study partner has consistent contact with the participant and, in the opinion of the investigator, is sufficiently knowledgeable about the participant's ongoing condition to provide accurate and current information. * The study partner has sufficient English-language proficiency to complete study partner assessments. * The study partner is willing and able to provide informed consent on their own behalf in accordance with ICH guidelines and applicable institutional regulations. * The study partner is, in the opinion of the investigator, reliable and competent; willing and able to accompany the participant to all study visits and comply with study procedures; reachable by telephone or email as needed; and sufficiently knowledgeable about the participant's ongoing condition(s) to provide accurate and current information regarding the participant's health and well-being. Exclusion Criteria: * The participant has at least one additional known genetic abnormality outside the human 15q11.2-q13.3 locus causing a probable or known developmental disability. * At least one standard-of-care treatment (medication or adjunctive therapy) used by the participant was changed during the 28 days (4 weeks) prior to the first study visit. Treatments include, but are not limited to, doses of anti-epileptic medications, behavioral management medications, sleep medications, gabapentin, cannabidiol, special diets, supplements, speech therapy, occupational therapy, applied behavioral analysis (ABA), psychosocial interventions, physical therapy, or nutritional support. * The participant has unstable epilepsy, defined as having an emergency department visit or hospitalization for seizure-related concerns within the 28 days (4 weeks) preceding the initial study visit. * The participant is of childbearing potential and is either pregnant, breastfeeding, or not using an adequate method of contraception; abstinence is acceptable. * The participant has a clinically relevant history of malignancy; clinically significant abnormal test results; clinically significant cardiovascular, hematologic, hepatic, muscular, neurologic, or renal disease; or has experienced other clinical events which, in the opinion of the investigator, render participation unsuitable. * The participant has a lifetime history of treatment with any cell- or gene-based therapy, including antisense oligonucleotides or gene-editing therapies. * The participant has received any investigational therapy other than a cell- or gene-based therapy within 28 days or 5 half-lives (whichever is longer) preceding the initial study visit. * The participant is currently enrolled or plans to enroll in an interventional study involving an investigational agent or device during the planned observation period. * The participant has a known contraindication to electroencephalography, actigraphy, or any other study procedure described in the schedule of assessments. * The participant or study partner is, in the opinion of the investigator, unsuitable for participation in any other way, including an inability to fulfill study requirements.