SMARCB1 and ataxia telangiectasia: BACKGROUND: Atypical teratoid/rhabdoid tumors (AT/RT) are aggressive pediatric CNS malignancies characterized by SMARCB1 loss, which leads to the dysregulated expression of Enhancer of Zeste Homolog 2 (EZH2), a key catalytic component of the Polycomb Repressive Complex 2 (PRC2).