CYP2B6 and neoplasm: Purified protein standards (CYP2B6, CYP2S1, and CYP3A4), MT-3 cells lysate (AKR1C2), human liver lysate (AKR1C1 and CBR1), and a pool of tumor samples (AKR7A3) were used as a calibrator for comparison of variability among membranes.