Inclusion Criteria: * Participant has a confirmed diagnosis of classical/typical Rett Syndrome with a documented mutation of the MECP2 gene that results in loss of function. * Participant is between ≥5 to ≤8 years of age at the time of consent. * Participant must be up to date with all relevant local vaccination requirements, with last vaccination dose received at least 42 days prior to the start of the immunosuppression regimen. * Participant's parent/caregiver must be willing to allow participant to receive blood or blood products for the treatment of an AE if medically needed. Exclusion Criteria: * Participant has another neurodevelopmental disorder independent of the MECP2 gene loss of function mutation, or any other genetic syndrome with a progressive course. * Participant has a history of brain injury that causes neurological problems. * Participant had grossly abnormal psychomotor development in the first 6 months of life. * Participant has a diagnosis of atypical Rett syndrome. * Participant has an MECP2 mutation that does not cause Rett syndrome. * Participant requires non-invasive and invasive ventilatory support. * Participant has contraindications for IT administration of TSHA-102 or lumbar puncture procedure, other medical conditions, or contraindications to any medications required for IT administration. * Participant has acute or chronic hepatitis B or C infections.