RecruitingDrug trialATTUNE: A Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of Intrathecally-Administered ION440 in Participants With Methyl CpG Binding Protein 2 (MECP2) Duplication Syndrome (MDS)The primary purpose of this study is to evaluate the safety and tolerability of ION440.