Molecular endophenotypes represent a plausible strategy to summarize complex NPDs with high clinical heterogeneity, by separating them into different subtypes with similar pathogenic mechanisms based on DNMs, such as MECP2 variants in patients with Rett syndrome (Amir et al., 1999; Tillotson et al., 2017). Here, MECP2 is linked to atypical Rett syndrome.