The coexpression of Pax5+/− and Stat5b-CA (a constitutively active form of STAT5) induce ALL in 100% of the transgenic mice [32], indicating that Stat5 activation is the necessary second hit that synergize with Pax5 heterozygosity and fully transforms the preleukemic Pax5+/− cells. This evidence concerns the gene STAT5B and acute lymphoblastic leukemia.