In vitro cytotoxicity to zebularine. (A) Dose-response curves showing the in vitro cytotoxic response to zebularine in individual leukemia cell lines with or without MLL rearrangements or (B) the mean cytotoxic response for MLL-rearranged ALL cell lines (n = 4) and for the other ALL cell lines (n = 4). Error bars represent SEM. The differences between the means of the groups were statistically analyzed by the use of the 2-tailed Student t test (P < .01 for each concentration used).