CTLA4 and B-cell chronic lymphocytic leukemia: Although the median percentages of Ki67-positive cells did not significantly differ between the studied groups of CLL patients at each time point tested (Table 3), and in the low CTLA-4 expressors the median proportions of Ki67+ leukaemic cells following ex vivo stimulation were comparable to the corresponding cells in healthy individuals, we observed that in the high CTLA-4 expressors, the median frequency of Ki67-positive leukaemic cells after 72 h of stimulating culture was markedly lower than in healthy controls (Table 3).