JAK2 and precursor B-cell acute lymphoblastic leukemia: B‐ALL tumors from the lymph nodes of Cd79a‐Cre Ikzf1neo/+Pax5LSL‐Jak2/+ mice had a similar cell surface phenotype as Pax5Jak2/+ B‐ALL tumors except for higher expression of B220 and CD19 (Fig 3H), although they still lost the wild‐type Pax5 allele (Appendix Fig S3B).