SMARCB1 and ataxia telangiectasia: Atypical teratoid rhabdoid tumor (AT/RT) is a rare embryonal central nervous system (CNS) cancer diagnosed most often on the basis of biallelic loss of SMARCB1 (SWI/SNF Related, Matrix Associated, Actin Dependent Regulator Of Chromatin, subfamily B, Member 1), a master chromatin regulator which is essential during development1.