Ibrutinib is the first-generation BTKi, although it is effective in the treatment of MCL, it is still discontinued or reduced due to significant side effects.[4] Because of its structure, Ibrutinib binds to cysteine-utilizing kinases, such as epidermal growth factor receptor, steroid receptor coactivator kinase, B-lymphocyte kinase (BLK), tyrosine kinase (TEC), bone marrow tyrosine kinase gene (BMX), and ITK.[24] Off-target effects of this irreversible BTKi on these kinases are thought to be associated with adverse drug reactions. Here, ITK is linked to mantle cell lymphoma.