A Novel, Regulated Gene Therapy (NGN-401) Study for Females With Rett Syndrome

Inclusion Criteria: * Females who are between the ages of ≥4 and ≤10 years for Arms 1 and 2 (Arms closed). Females who are ≥11 years of age or older for Arm 3 (Arm closed). Females who are ≥3 for Arm 4, the pivotal cohort. * Diagnosis of typical Rett syndrome with a documented disease-causing mutation in the methyl-CpG-binding protein 2 (MECP2) gene * Current anti-epileptic drug regimen has been stable for at least 12 weeks * Participant must be in the post-regression stage * Participant and caregiver should reside within a 2-hour drive of the study center for at least 3 months following treatment * Participant must have never taken trofinetide or have taken trofinetide and discontinued due to tolerability, lack of efficacy, or other reasons. Following NGN-401 dosing, trofinetide may be initiated after a specified time period and with the support of the treating clinician. Exclusion Criteria: * Normal or near normal hand function * Has a current clinically significant condition other than Rett syndrome * Presence of a concomitant medical condition that precludes intracerebroventricular administration, or use of anesthetics or immune suppression needed for study related procedures Other inclusion and exclusion criteria apply.