This is a Phase 1/2, first in human, open-label, dose-escalation study to evaluate the safety, tolerability, and clinical activity of a single dose of JAG201 administered via intracerebroventricular (ICV) injection in pediatric and adult participants with SHANK3 haploinsufficiency resulting from SHANK3 loss of function mutations and chromosomal deletions encompassing the SHANK3 gene. Clinical data will be evaluated for safety, tolerability, and preliminary clinical activity of JAG201 in pediatric and adult participants with SHANK3 haploinsufficiency. The pediatric cohorts will start enrolling first and the enrollment for adult cohorts may be initiated at a later timepoint in the study.