BTK and B-cell chronic lymphocytic leukemia: The first-in-class BTKi ibrutinib, which inactivates BTK by binding covalently to cysteine 481 in the ATP-binding site of the enzyme in an irreversible manner, is FDA-approved for the treatment of CLL, mantle cell lymphoma (MCL), Waldenström’s macroglobulinemia, marginal zone lymphoma, and graft-versus-host disease.