Correlation between Bcl-2 family proteins and in vitro sensitivity to fludarabine. Samples from 112 CLL patients were evaluated for in vitro sensitivity to fludarabine. LD₅₀ values, the concentration of fludarabine required to kill 50% of the cells in culture, were then plotted against Bcl-2 family protein expression levels as continuous variables. (A) Bcl-2 expression showed no correlation, (B) Bax was negatively correlated, and (C) Mcl-1 was positively correlated with in vitro sensitivity to fludarabine. (D) Bcl-2/Bax ratios and (E) Mcl-1/Bax ratios showed even stronger correlations with in vitro drug responses.