Figure 6b shows a representative histogram demonstrating the reduction in Ki67 in CLL cells from ibrutinib treated mice compared to control. Ibrutinib significantly inhibited tumor proliferation as reflected in decreased Ki67 expression in CLL cells in both the PB and spleen of ibrutinib treated mice (mean reduction >50%, Figure 6c-e; P≤.006). Thus ibrutinib potently and selectively inhibits tumor proliferation in vivo. This evidence concerns the gene MKI67 and B-cell chronic lymphocytic leukemia.