ADA2: adenosine deaminase 2; AT/RT: atypical teratoid/rhabdoid tumor; BMT: bone marrow transplant; CNS: central nervous system; CTCAE: Common Terminology Criteria for Adverse Events; DLBCL: diffuse large B-cell lymphoma; HLH: hemophagocytic lymphohistiocytosis; IL2RG: interleukin-2 receptor gamma; ITK: IL-2-inducible T-cell kinase; MALT1: mucosa-associated lymphoid tissue lymphoma translocation protein 1; PIDD: primary immunodeficiency disorder. The gene discussed is ITK; the disease is hemophagocytic syndrome.