For example, both deletion and duplication lead to neurodevelopmental phenotypes of Rett syndrome and MECP2 duplication syndrome (Ramocki et al., 2009); the FXS-related gene, FMR1 (Auerbach et al., 2011; Hickey et al., 2013; Arsenault et al., 2016); and the 16p11.2 CNV (Horev et al., 2011; Niarchou et al., 2019). The gene discussed is MECP2; the disease is fragile X syndrome.