Tail-vein injection of Jak1(–/–) cells results in a more severe form of leukemia than injection of Jak1(+/–) cells. A total of 3 × 10⁶ tumor cells were injected into nude mice via the tail vein, and the mice were analyzed 25 days thereafter. Four nude mice that received PBS only were used as controls. Data show mean values ± SD (n = 6 for tumor cell injections and n = 4 for control mice). (A) Spleen weight of the mice at the time point of analysis. (B) White blood cell count and (C) number of CD19⁺ cells in the bone marrow at the time point of analysis. (D) Blood smear of one Jak1(–/–)– and one Jak1(+/–)–injected nude mouse. Hematoxylin and eosin stain, original magnification, × 100.