While the 4‐fold lower expression of the Pax5‐Jak2‐KD protein in pro‐B cells is expected to significantly delay the tumor onset, the complete absence of B‐ALL in 1‐year‐old Pax5Jak2‐KD/+ mice nevertheless indicates a critical role of the Pax5‐Jak2 kinase activity in the initiation of leukemia development. Here, JAK2 is linked to acute lymphoblastic leukemia.