Methyl-CpG binding protein 2 (MECP2) is a dosage-sensitive, X-linked gene critical for central nervous system development and functional maintenance, which gain-of-function causes MECP2 duplication syndrome (MDS). Affecting primarily in males, this disorder is characterized by severe intellectual disability, motor dysfunction, infantile hypotonia, epilepsy, respiratory tract infections, and premature death before 25 years of age with no curative therapy. HG204 is a CRISPR RNA-editing therapy packaging novel high-fidelity Cas13Y (hfCas13Y) technology, using one single adeno-associated virus (AAV) vector to target and knock down MECP2 mRNA in the brain. Preclinical studies showed that a single intracerebroventricular injection of HG204 persistently decreased MECP2 mRNA and MECP2 protein in the cortex of the MDS mice, reversed the abnormal motor and social phenotypes, and significantly prolonged survival in MDS mouse models.