STAT5B and acute lymphoblastic leukemia: Finally, B‐ALL development requires at least two cooperating mutations, as exemplified by the constitutive activation (ca) of STAT5 combined with heterozygous loss of Pax5 in transgenic caStat5a Pax5+/− and caStat5b Pax5+/− mice (Heltemes‐Harris et al, 2011; Smeenk et al, 2017).