AT-rich interactive domain 1B, ARID1B, and genes in the chromatin modification complex, SWItch/Sucrose Non-Fermentable (SWI/SNF), are thought to cause Coffin–Siris syndrome (CSS) [6], an exceedingly rare neurodevelopmental disorder with fewer than 200 individuals diagnosed [7]. The gene discussed is ARID1B; the disease is neurodevelopmental disorder.