The frequency of YFP+ leukaemia cells in the peripheral blood of the tamoxifen-induced HCK−/− group was lower than that in the peripheral blood of the HCK+/+ group (Fig. 4c), which was consistent with a decrease in the sizes and weights of the spleens of HCK−/− leukaemic recipient mice (Fig. 4d, e, f). This evidence concerns the gene HCK and leukemia.