In about 5% of all cases, it is associated with syndromic forms of ASD such as fragile X syndrome (FXS; FMR1 mutation), Rett syndrome (RTS; MECP2 mutation), Angelman syndrome (15q11q13 deletion), Timothy syndrome (CACNA1C mutation), Phelan-McDermid syndrome (22q13 deletion) and Kleefstra syndrome (EHMT1 mutation) [2]. The gene discussed is CACNA1C; the disease is Rett syndrome.