In a phase I open-label, dose-escalation study of the first-in-class covalent Bruton’s tyrosine kinase inhibitor (BTKi) ibrutinib in relapsed/refractory (R/R) B-cell non-Hodgkin lymphoma (NHL) and CLL, the level of ibrutinib occupancy of BTK was used as a biomarker when determining the recommended phase II dose [12]. The gene discussed is BTK; the disease is B-cell chronic lymphocytic leukemia.