High-level MCL-1 expression is associated with in vitro prednisolone resistance in both MLL-rearranged infant ALL and noninfant pediatric precursor B-ALL. Distribution of the relative MCL-1 expression as determined by quantitative real-time RT-PCR (TaqMan) analyses among patients with (A) MLL-rearranged infant ALL and (B) noninfant (children > 1 year of age) B-ALL. Dotted lines either indicate the mean or median expression among both patient groups. Among the MLL-rearranged infant ALL cases, the median and mean MCL-1 expressions coincided. The median or mean MCL-1 expression level was used as the cutoff between patients, dividing the patients into groups displaying “high” or “low” MCL-1 expression. Mean cytotoxic dose-response curves for prednisolone in patients characterized by high or low MCL-1 expression are shown for patients with (C) MLL-rearranged infant ALL and (D) noninfant ALL, respectively. Differences in mean cytotoxicity responses between patient groups were statistically analyzed using the 2-tailed Student t test. Error bars represent SEM. *Indicates significant differences between the means of the groups at P < .05.