An 11-year-old male patient with the deletion of <i>IKZF1</i> (Ikaros family zinc finger 1) and positive Breakpoint Cluster Region-C-Abelson oncogene 1(<i>BCR-ABL1</i>) acute lymphoblastic leukemia developed mucositis, gastrointestinal toxicity, hepatotoxicity, myelosuppression, and severe dermatologic toxicity during the first and second courses of high-dose methotrexate. Here, IKZF1 is linked to acute lymphoblastic leukemia.