Membranous expression of BAFF and APRIL on B-CLL leukemic B cells. B cells purified from healthy blood donors (n = 6) or B-CLL patients (n = 15) were tested using immunofluorescence and flow cytometry for surface expression of BAFF and APRIL by labeling with specific antibodies or isotype control, and the percentages of positive cells were estimated using the Kolmogorov-Smirnov test, as described in “Patients, materials, and methods.” (A) Representative histograms of 3 healthy donors and 3 B-CLL patients. Open histograms indicate isotype control; gray histograms, BAFF (left) or APRIL (right) labeling. (B) Global distribution of BAFF- and APRIL-positive cells in healthy donors and B-CLL patients. Percentages for normal B lymphocytes: BAFF, mean 8.8% (range, 4%-14%); APRIL, mean 10% (range, 2%-21%). Percentages for B-CLL B lymphocytes: BAFF, mean 34.1% (range, 14%-71%) (P = .0001); APRIL, mean 35.5% (range, 13%-53%) (P = .0001).