Ibrutinib, an irreversible inhibitor of Bruton’s tyrosin kinase (BTK) and IL-2 inducible T-cell kinase (ITK) inhibited not only the generation of human MDSCs in vitro but also the recruitment of CD11b+/Gr1+ MDSCs in the tumor and spleen in murine breast cancer and melanoma models. The gene discussed is BTK; the disease is breast cancer.