ARID1B is by far the most frequently mutated gene (51–75%) (refs. 4–6) in Coffin Siris syndrome (CSS) (OMIM 135900) and large-scale exome sequencing studies invariably find that pathogenic variants in ARID1B are among the most frequently identified causes in unspecified ID cohorts (usually around 1%) (refs. 1,2). This evidence concerns the gene ARID1B and Coffin-Siris syndrome.