Bruton's tyrosine kinase (BTK) inhibitor ibrutinib has shown extremely positive results in the treatment of chronic lymphocytic leukaemia (CLL; Burger et al., 2015; Byrd et al., 2014) and is now being tested in combination with other therapies (Collett et al., 2017; Hillmen et al., 2019; Kater et al., 2019). The gene discussed is BTK; the disease is B-cell chronic lymphocytic leukemia.