Patient K8 was ultimately diagnosed with Coffin–Siris syndrome with a de novo novel variant, c.5547delC (p.L1850*), in ARID1B and patient K12 harbored a de novo novel variant, c.1453_1455delGCT (p.A485del), in SMC3, which is related to a mild variant of Cornelia de Lange syndrome with predominant ID. Here, SMC3 is linked to Coffin-Siris syndrome.