BTK and B-cell chronic lymphocytic leukemia: Ibrutinib, the first-in-class orally bioavailable BTKi, irreversibly binds to the cysteine 481 (C481) residue of BTK, thereby blocking the phosphorylation of downstream kinases in the BCR cascade, inducing modest apoptosis in CLL cells, independent of microenvironmental survival signals [52,53].