SMARCB1 and neoplasm: Almost 100% of rhabdoid tumors are defined by loss of SMARCB1 (SWI/SNF Related, Matrix Associated, Actin Dependent Regulator of Chromatin, Subfamily B, Member 1), a tumor suppressor gene that encodes the SNF5 subunit of the SWI/SNF (Switch/Sucrose Non-Fermentable) chromatin remodeling complex.