Correlation of drug resistance with Fas-mediated resistance as determined by IC₅₀ for drug- versus Fas-induced cell death. The percentage of cell death induced by anti-Fas was assayed by flow cytometry analysis of 7-AAD cells. IC₅₀ of the selection agent is determined by the concentration that inhibits growth by 50% in a modified tetrazolium-based assay as previously described.30 (A) Doxorubicin-resistant cells (Dox); (B) Mitoxantrone-resistant cells (MR).