In vitro sensitivity of ALL cells to dexamethasone and vincristine. Sorted populations from 10 patients (2, 3, 5, 7, 9, 11, 14, and 19-21) were cultured with dexamethasone for 48 hours (A). Cell viability was assessed by annexin V–propidium iodide staining. Data are mean plus or minus SE. * CD133⁺/CD19⁻ cells were significantly less sensitive to treatment with dexamethasone than the CD133⁺/CD19⁺ and the unsorted bulk leukemia population (P < .01). Cells from patients 5, 11, 14, and 20 to 22 were cultured with vincristine for 72 hours and the viability assessed in the same way (B). The percentage viable is represented relative to untreated controls. Subsequently, growth of drug-treated CD133⁺/CD19⁻ cells from patients 5, 14, 20, 21, and 22 was evaluated in suspension culture (C). The cultures were maintained with weekly half-media changes, and the absolute number of viable cells present at each week was determined using flow cytometry. The results for the 5 patients are shown, each represented by a different symbol.