STAT5B and acute promyelocytic leukemia: The fusion partner also plays a key role in the response to the retinoid treatment: APL patients carrying NPM1 and NuMA fusion partners respond clinically to ATRA treatment [40, 41], whereas APL cases involving PLZF (promyelocytic leukemia zinc finger), IRF2BP2 (interferon regulatory protein 2 binding protein 2) and STAT5b presented with ATRA resistance and a poor prognosis [42–45].