BTK and plasma cell myeloma: The oral irreversible (by virtue of its covalent binding to Cys-481 in the ATP binding pocket) BTK inhibitor ibrutinib (Figure 7), which has been approved for the treatment of CLL, mantle cell lymphoma (MCL), and Waldenstrom’s macroglobulinemia among others, also has been studied for possible application in MM owing to reports of robust expression of BTK in myeloma cells [179].