MTT assays showing broad-spectrum single-agent obatoclax cytotoxicity in primary infant ALL. (A) Surviving fraction plots of MTT assay data by MLL subsets in diagnostic leukemia samples from 54 cases of primary infant ALL (n = 52) or infant bilineal acute leukemia (n = 2). Cells were treated for 72 hours with increasing obatoclax concentrations. Experiments were performed 1-3 times per patient sample (3-6 replicates/condition per experiment). (B) Box and whisker plots of EC₅₀ values ascertained by inhibitory sigmoid E_max models of MTT assay data in 54 cases shown in (A). Significant differences between EC₅₀ values were determined by Wilcoxon’s test (Table 1). (C) Kaplan-Meier curves showing similar EFS among infants with pretreatment diagnostic ALL samples in obatoclax-sensitive and obatoclax-resistant categories defined using a 176-nM cutoff. (D) Surviving fraction plot of MTT assay data for PBMCs after 72 hours of treatment with increasing obatoclax concentrations; assay was performed twice (1-3 replicates/condition per experiment).