Rett syndrome in the therapeutic era: Rethinking diagnosis, boundaries, clinical outcomes, and financing.

Rett syndrome (RTT) is a severe neurodevelopmental disorder characterized by multiple neurological impairments, which affects predominantly females. It is associated, in most cases, with pathogenic variants in MECP2, the gene encoding the methyl-CpG-binding protein 2. In the four decades since its description in the English literature, the RTT field has seen major progress in the understanding of its genetics, neurobiology, and clinical evolution. These have led to the first drug approved for core symptoms of a neurodevelopmental disorder, trofinetide, and gene replacement programs at advanced development. These achievements have brought new challenges and highlighted unresolved issues. These include diagnosis before developmental regression and of individuals without core RTT features, including males; the role of MECP2 testing in RTT diagnosis; the boundaries of RTT; instruments needed for assessing clinical evolution in research and practice; the definition of meaningful clinical improvement; and the development, availability, and affordability of new treatments. These topics are reviewed in terms of their implications for RTT and for other genetic neurodevelopmental disorders.