Ibrutinib is an irreversible inhibitor of Bruton tyrosine kinase (BTK) that binds covalently to the cysteine residue (C481) in the kinase domain. This inhibition has been shown in vitro to induce modest CLL cell apoptosis and to abolish proliferation and B-cell receptor (BCR) signaling. Clinical trial results with this agent have been outstanding, including an estimated 26-month progression-free survival (PFS) of 75% for patients with relapsed and refractory disease. Although PFS with ibrutinib is excellent, the overall response rate for this group of relapsed patients is only 71%, lagging behind the clinical benefit seen in 88% of patients because of lymphocytosis induced by this agent and all agents targeting the BCR pathway.