Notably, the B220low B cells in the bone marrow of 4‐week‐old Pax5Jak2/+ mice already expressed CD93, IL‐7Rα, and Flt3 (Fig 1J) and thus had a similar cell surface phenotype as the B‐ALL tumors in the lymph nodes of Pax5Jak2/+ mice (Fig 1I). Here, FLT3 is linked to acute lymphoblastic leukemia.