STAT5A and acute lymphoblastic leukemia: The therapeutic effects of enzymatic Jak2 inhibition in Eμ-Crlf2/Jak2R683G and Eμ-Crlf2/Jak2P933R B-ALL cells assessed using JAK2i ruxolitinib also revealed a relatively small but significant survival benefit that corresponded with a reduction in splenic burden at terminal disease, decreased activity of Stat5, and concomitant Jak2Y1007/1008 hyperphosphorylation (Fig. 3G,H; Supplemental Fig. S3D–G).