An irreversible inhibitor of BTK (IC50 = 0.5 nM) that selectively blocks B cell activation, promoting apoptosis and preventing homing to the protective tumor microenvironment, at concentrations that do not affect T cell receptor signaling (1,000-fold more potent); studied clinically for the treatment of diseases associated with B cell antigen receptor signaling, including MCL, CLL, and non-Hodgkin.