MECP2 mutations are ideal in this regard, as Rett syndrome is the result of loss-of-function mutations in MECP2, while duplication of the MECP2 gene causes a distinctive syndrome, the MECP2 duplication syndrome, that shares autism symptoms with Rett (OMIM: 3000053; Ramocki et al., 2009; Lombardi et al., 2015; Leonard et al., 2017). Here, MECP2 is linked to atypical Rett syndrome.