Gene expression profiling identifies high-level MCL-1 expression as a common marker for prednisone resistance among patients with childhood ALL. (A) The top 50 up-regulated probe sets correlate to either prednisone resistance or sensitivity in noninfant (> 1 year; n = 27) and infant (< 1 year; n = 25) patients with pediatric ALL. Raw microarray data can be found in supplemental Table 1. (B) Classification accuracy of prednisolone-sensitive or -resistant infant ALL or noninfant ALL samples on the basis of their gene expression profile. The error rate in class prediction is plotted against the number of probe sets used to build the model. (C) The overlapping genes associated with prednisone resistance in both the infant and noninfant ALL signatures. Merging the signatures shows that high-level MCL-1 expression is common to both noninfant and infant patients with ALL who are resistant to prednisone in vitro.