Our results indicate that ER stress is in fact triggered in these types of multidrug-resistant colon cancer cells upon exposure to acetylated androstano-arylpyrimidines and that these novel steroids are capable of inhibiting ABC transporter activity, thereby attenuating drug resistance and consequently sensitizing MDR cancer cells to drug-induced apoptosis. This evidence concerns the gene ABCG2 and malignant colon neoplasm.