ARID1B and Coffin-Siris syndrome: The most well defined BAFopathy, caused by pathogenic variants in several BAF subunit genes including ARID1A, ARID1B, SMARCA4, SMARCC2, SMARCB1, SMARCE1, DPF2, and BICRA, is the autosomal dominant Coffin-Siris syndrome (CSS; MIM 135,900) (Hoyer et al. 2012; Santen et al. 2013; Kosho et al. 2014b; Vasileiou et al. 2018; Barish et al. 2020; Vasko et al. 2021; Bosch et al. 2023).