PMCH and atypical Rett syndrome: mCH has been established as an important base modification with likely biological functions during mammalian brain development (Lister et al., 2013), and links to Rett syndrome pathogenesis (Chen et al., 2015; Gabel et al., 2015; Boxer et al., 2020; Lavery et al., 2020), there are still many unknowns related to its regulation and function.