BTK and B-cell chronic lymphocytic leukemia: Ibrutinib, a first-in-class BTK inhibitor, was first approved in the US in 2013 for the treatment of relapsed or refractory (R/R) mantle cell lymphoma (MCL) and has been a standard of care for naïve and R/R chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), R/R MCL, and R/R Waldenström macroglobulinemia (WM).