To gain further insight into the nuclear function of Pax5‐Jak2, we next studied the subcellular localization of Pax5‐Jak2 by nuclear‐cytoplasmic cell fractionation and immunoblot analysis, which revealed that the Pax5‐Jak2 protein was almost exclusively present in the nucleus of Pax5Jak2/+ B‐ALL cells (Appendix Fig S6A). Here, JAK2 is linked to acute lymphoblastic leukemia.