Since neutropenia is an on-target toxicity of BTK inhibitors and ibrutinib and zanubrutinib inhibit BTK via the same mechanism, the higher incidence of grade ≥ 3 neutropenia among zanubrutinib recipients can be attributed, at least in part, to its greater bioavailability as measured by relative plasma exposures for the two agents [15]. Here, BTK is linked to neutropenia.