Upon loss of full‐length Pax5, the Pax5‐Jak2 protein was, however, able to interact with most genomic Pax5‐binding sites in Pax5Jak2/+ B‐ALL cells, strongly indicating that Pax5‐Jak2 functions as DNA‐binding oncoprotein in the nucleus. This evidence concerns the gene JAK2 and acute lymphoblastic leukemia.