Diagnosis integrates clinical evaluation (including detailed family history), immunohistochemistry demonstrating loss of SMARCB1 or SMARCA4 protein expression, and tumor sequencing for somatic alterations; germline testing for pathogenic variants in SMARCB1 or SMARCA4 is recommended for all patients with ATRT. The gene discussed is SMARCA4; the disease is atypical teratoid rhabdoid tumor.