CSS is characterized by “developmental or cognitive delay, hypotonia, sparse scalp hair, distinctive facial features, aplasia or hypoplasia of the distal phalanx or nail of the fifth and additional digits, and hypertrichosis.”7 Approximately 70 ARID1B-CSS patients4–6,8–12 have been described. The gene discussed is ARID1B; the disease is Coffin-Siris syndrome.