SHANK3 and Pelizeaus-Merzbacher spectrum disorder: Despite mutations in SYNGAP1 and SHANK3 resulting in opposite effects on E/I ratio, the phenotypic presentations of both SYNGAP1-ID and PMD are similar in that they can include epilepsy, intellectual disability (ID), autism spectrum disorder (ASD), severe expressive and receptive speech delay, hypotonia, sleep abnormalities, global developmental delays, and behavioral issues (3, 5, 8, 11–14).