Phase 3 Efficacy and Safety Study of GTX-102 in Pediatric Subjects with AS

Inclusion Criteria:

• Signed informed consent from parent(s) or legal guardian(s)

• Confirmed diagnosis of AS with genetic confirmation of full maternalubiquitin-protein ligase E3A (UBE3A) gene deletion causing AS in the region of 15q11.2 q13

• Able to ambulate independently, or with assistance at the Screening Visit (note, achild whose primary means of mobility is by wheelchair is excluded from the study)

• Platelet count, prothrombin time / international normalized ratio, and partialthromboplastin time within 1.5x the normal limits at the Screening Visit

• Willing and able to comply with scheduled visits, drug administration plan,laboratory tests, and all study procedures, including LP procedure and toleratinganesthesia without intubation

• From the time of informed consent through to at least 6 months after the final doseof GTX-102, females of childbearing potential who are sexually active must usehighly effective contraception or abstinence. Males are able to participate if theyagree to remain abstinent (refrain from heterosexual intercourse) or use acceptablecontraceptive methods during the study and for at least 3 months after the finaldose of GTX-102

Exclusion Criteria:

• Any change in medications or diet/supplements intended to treat symptoms of AS (eg,sleeping aids, antiseizure medications, supplements, dietary change includingketogenic or low-glycemic index diet, other) within the month prior to the ScreeningVisit (excluding weight-based adjustments)

• Any condition that creates an increased risk of unsuccessful LP

• Current or expected concomitant use of drugs that increase the risk of bleeding (eg,heparin, low molecular weight heparin, platelet inhibitors)

• Known hypersensitivity to GTX-102 or its excipients that, in the judgment of theInvestigator, places the subject at increased risk for adverse effects

• Presence or history of any condition that, in the view of the Investigator, wouldinterfere with participation, pose undue risk, or would confound interpretation ofresults

• Any clinically significant cardiovascular, endocrine, hepatic, renal, pulmonary,gastrointestinal, neurologic, malignant, metabolic, psychiatric, or other conditionor infection that, in the judgment of the Investigator, will pose a safety risk,make the subject unsuitable for participation in, and/or unable to complete thestudy procedures

• Any laboratory abnormality, that, in the Investigator's opinion, could adverselyaffect the safety of the subject, make it unlikely that the course of treatment orfollow up would be completed, or impair the assessment of study result

• Pregnant or breastfeeding or planning to become pregnant (self or partner) at anytime during the study

• Use of any investigational product or investigational medical device within 6 monthsor 5 half-lives prior to the Screening Visit or any prior use of gene therapy or ASOregardless of duration since last administration

• Concurrent participation in any study, including observational natural historystudies