(A) Drug resistance is associated with α₄expression in melphalan-resistant (8226/LR5) and revertant (LR5ood) cell lines. 8226/LR5 were maintained in 5 × 10⁻⁵ mmol/L melphalan (LPAM) and LR5ood were maintained out of drug for 20 weeks. ₄ expression was measured by flow cytometry and drug resistance was measured by MTT cytotoxicity analysis. Resistance values are reported as the IC50 dose of LPAM relative to 8226/S. ₄ expression levels and melphalan resistance levels of 8226/LR5 were found to be higher than 8226/S (P < .05). ₄ expression and melphalan resistance of LR5ood were found to be equal to those of the 8226/S parent line. Bars are the SD of three different experiments. (B) Drug resistance is associated with ₄ expression in doxorubicin-resistant (8226/DOX6) and revertant (DOX6ood) cell lines. 8226/DOX6 were maintained in 6 × 10⁻⁸ mol/L doxorubicin and DOX6ood were maintained out of drug for 20 weeks. ₄ expression was measured by flow cytometry and drug resistance was measured by MTT cytotoxicity analysis. Resistance values are reported as the IC50 dose of doxorubicin relative to 8226/S. ₄ expression levels and doxorubicin resistance levels of 8226/DOX6 were found to be higher than 8226/S (P < .05). ₄ expression and doxorubicin resistance of DOX6ood were found to be equal to those of the 8226/S parent line. Bars are the SD of three different experiments.