Expression of A1 leads to resistance to chemotherapy. The indicated cell lines were incubated for 24 hours in the indicated concentration of drug (panel A, cytarabine [Ara-C]; panel B, doxorubicin), and surviving cells quantified using an MTT assay. Results were normalized to the value obtained with no chemotherapy. Error bars represent standard error of the mean of triplicate assays. (C) Annexin V staining was performed with the same cell lines treated with 1 μM Ara-C as described. Apoptotic cells were defined as those which were Annexin V positive and negative for propidium iodide.