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 ≥3for Arm 4, the pivotal cohort.

• Diagnosis of typical Rett syndrome with a documented disease-causing mutation in themethyl-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 centerfor at least 3 months following treatment

• Participant must have never taken trofinetide or have taken trofinetide anddiscontinued due to tolerability, lack of efficacy, or other reasons. FollowingNGN-401 dosing, trofinetide may be initiated after a specified time period and withthe 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 intracerebroventricularadministration, or use of anesthetics or immune suppression needed for study relatedprocedures

Other inclusion and exclusion criteria apply.