In vitro tipifarnib sensitivity of untreated ALL cells and N BM cells as measured with a total cell-kill assay. Results are depicted in LC₅₀ values (μg/mL), the concentration of tipifarnib needed to kill 50% of the cells. Each symbol represents the LC₅₀ value of an individual sample; the horizontal line depicts the median LC₅₀ value. Within AML, samples classified as FAB M5 AML are more sensitive to tipifarnib in vitro than other AML samples (P = .021). Within ALL T-ALL samples are more sensitive to tipifarnib in vitro than BCP ALL samples (P = .029). BCP ALL and N BM cells are relatively resistant to tipifarnib in vitro when compared to AML blasts (BCP ALL versus AML P = .01, N BM versus AML, P < .001). ^*Statistically significant at P < .05 level. Non-FAB M5 indicates all FAB types other than FAB M5.