BTK and B-cell chronic lymphocytic leukemia: Two ongoing randomized studies of zanubrutinib versus ibrutinib, one in treatment-naïve and one in relapsed/refractory CLL/SLL patients (NCT03053440 and NCT03734016), aim to determine whether consistent, continuous BTK blockade with a more selective inhibitor results in fewer off-target effects and translates into improvements in disease control.