JAK2 and acute lymphoblastic leukemia: Consistent with the absence of wild‐type Pax5 mRNA, Pax5Jak2/+ B‐ALL cells failed to express full‐length Pax5 protein, in contrast to the Pax5‐Jak2 protein, as shown by immunoblot analysis with a Pax5 paired domain‐specific antibody (Fig 3B).