IBTK and mantle cell lymphoma: Resistance to these BTKi in MCL is often related to point mutations affecting the BTKi binding site, cysteine 481 (C481), and is less commonly related to other mechanisms of resistance, such as activating mutations in the nuclear factor kB (NF-kB) pathway and gain-of-function mutations in the downstream signaling substrates, such as phospholipase Cy2 (PLCy2) [27].