AXL and B-cell chronic lymphocytic leukemia: Axl expression: Next we assessed the levels of surface Axl expression on CLL B-cells obtained from 26 CLL patients (7 female, 19 male) who were on ibrutinib therapy for relapsed/refractory CLL or who had progressed while on ibrutinib treatment, by flow cytometry using a specific antibody to Axl [16].