JAK2 and neoplasm: Notably however, the expression of Pax5 from the heterologous Ikaros locus delayed but did not prevent B‐ALL development in Pax5Jak2/+ mice, indicating that the Pax5‐Jak2 protein could still induce tumor development in the presence of full‐length Pax5 in the mouse, which is analogous to the situation observed with human PAX5‐JAK2+ B‐ALLs.