Whereas all 10 Pax5 exons were expressed in Pax5+/+ pro‐B cells, abundant expression was detected in Pax5Jak2/+ B‐ALL cells only from exon 1 to exon 5, which code for the N‐terminal paired domain present in the Pax5‐Jak2 fusion protein (Fig 3A). Here, JAK2 is linked to acute lymphoblastic leukemia.