Comparison of Western blot and FACS assay. (A) Dilution series of treated and untreated CLL cells on Western blot. Irradiated (5 Gy; 100%) cells were mixed with nonirradiated (0%) cells. (B) The intensity of the Western blot bands was quantified and compared with mean fluorescent intensities of the FACS assay. Linear regression shows a high correlation between both methods (left panel: p53, r² = 0.8836; right panel: p21, r² = 0.8159). (C) Discrimination between CD19-positive CLL cells and CD19-negative cells in the FACS assay is shown in an exemplary sample. The Western blot shows induction of p21 and p53. In the dissection of the contributing cellular component by FACS (CD19, p21, p53), it is clear that CLL cells showed impaired p53 up-regulation, whereas CD19-negative cells induced p53 after irradiation. Black line, p53 expression of nonirradiated sample; gray line, p53 expression of irradiated sample.