Genotype-phenotype clinical studies have suggested that ARID1B truncating mutations are generally linked to the mildest cases of CSS-related intellectual disability, including some individuals without intellectual disability90, whereas single amino acid mutations of the SMARCB1 protein are correlated with the most severe cognitive impairment and growth delay in CSS21,69,85. Here, SMARCB1 is linked to Coffin-Siris syndrome.