Inclusion Criteria: * Females between the ages of 12 and \<22 in Part A (closed) and females between the ages of 6 and \<22 in Part B (pivotal cohort). * Participant has a clinical diagnosis of classic/typical Rett syndrome with a documented pathogenic mutation of the methyl-CpG-binding protein 2 (MECP2) gene that results in loss of gene function. * Participants must be willing to receive blood or blood products for the treatment of an AE if medically needed. * Participants and parent/caregiver must agree to reside within easy access to the study site prior to the baseline visit and at least 3 months after TSHA-102 treatment Exclusion Criteria: * Participant has another neurodevelopmental disorder independent of the MECP2 loss-of-function mutation, or any other genetic syndrome with a progressive course. * Participant has a history of brain injury that causes neurological problems or had grossly abnormal psychomotor development in the first 6 months of life. * Participant has a diagnosis of atypical Rett syndrome or a MECP2 gene mutation that does not cause Rett syndrome. * Participant requires invasive ventilatory support. Note: Other protocol defined inclusion/exclusion criteria may apply