It is difficult to discern the contributions of different neuronal groups to pathogenesis, however, in part because genes driving disorders such as RTT, Fragile X syndrome, Angelman syndrome, and tuberous sclerosis (MECP2, FMRP, UBE3A, and TSC1 and TSC2, respectively) participate in fundamental cellular processes in multiple cell types (Crino, 2013; Darnell et al., 2011; Lyst and Bird, 2015; Mabb et al., 2011), and in part because network alterations always induce compensatory changes in the circuit (Turrigiano, 2011). The gene discussed is MECP2; the disease is fragile X syndrome.