The emergence of tumors resistant to anticancer agents has been recognized as one of the major obstacles to effective cancer chemotherapy. Recent studies have elucidated various mechanisms in this resistance, such as induction of drug efflux pumps, in particular certain of the ABC transporters. Experiments to predict the sensitivity of tumors to anticancer agents using DNA microarray and SNPs analyses are ongoing. The development of the new anticancer agent ST1571 and the emergence of tumors resistant to ST1571 indicate the urgent need for clinically available reversing agents.