As wild‐type pro‐B and pre‐B cells abundantly express Jak1 but not Jak2 (ImmGen database), ruxolitinib may mediate its effect by inhibition of the endogenous Jak1, transgenic Pax5‐Jak2, or both kinases in Pax5Jak2/+ B‐ALL tumors. Here, PAX5 is linked to acute lymphoblastic leukemia.