Any Coffin-Siris syndrome in which the cause of the disease is a mutation in the ARID1B gene. The disease is Coffin-Siris syndrome 1 (MONDO_0007617, a Monarch Disease Ontology id). Also known as: ARID1B-related BAFopathy, COFFIN-SIRIS syndrome 1, CSS1, MRD12, autosomal dominant mental retardation 12, hypertrichosis, hyperkeratosis, intellectual disability, and distinctive facial features, hypertrichosis, hyperkeratosis, mental retardation, and distinctive facial features, intellectual disability, autosomal dominant 12.