STAT5B and cancer: Collectively, these findings indicate that the inhibitory effect of IST5 on Stat5 phosphorylation in cancer cells is not due to inhibition of Jak2 or other key kinases including Jak1, Jak3, Tyk2, Src, Lck, Fyn, TrkB/C, Abl1 or Abl2 (Table 1, Figure 5D).