While we could not find evidence for a nuclear role of Pax5‐Jak2 as an epigenetic regulator, high levels of active phosphorylated STAT5 and increased expression of STAT5 target genes were seen in Pax5Jak2/+ B‐ALL tumors, implying that nuclear Pax5‐Jak2 phosphorylates STAT5. Here, STAT5B is linked to precursor B-cell acute lymphoblastic leukemia.