MECP2 and atypical Rett syndrome: In Rett syndrome (RTT), a neurodevelopmental ASD due primarily to mutations in the methyl-CpG binding protein 2 (MECP2) gene, hiPSC-derived neural cells show reduced soma size, dendrite spine density, differentiation, and reduced spontaneous Ca2+ transient frequency in neurons and premature astroglial [104–106, 151, 152], as shown in human postmortem analyses.