In mouse, T‐ALL‐activating NOTCH1 mutations usually collide with loss‐of‐function mutations in Ikzf1 gene that encodes the zinc‐finger transcription factor IKAROS that regulate hematopoietic stem cell functions and drive lineage fate decisions of early hematopoietic progenitors. This evidence concerns the gene IKZF1 and acute lymphoblastic leukemia.