BTK and B-cell chronic lymphocytic leukemia: Launched in 2013, ibrutinib is the world's first commercially available BTK inhibitor and has been approved by FDA for 6 indications: chronic lymphocytic leukemia, small lymphocytic lymphoma, mantle cell lymphoma, Waldenstrom's macroglobulinemia, graft-versus-host disease, and marginal zone lymphoma [32–34].