There is now a range of cytotoxic drugs that have considerable clinical usefulness in producing responses in tumors and even, in a small proportion of cases, cure. However, the acquisition of drug resistance is a major clinical problem and is perhaps the main limiting factor in successful treatment of cancer. Thus, a tumor initially sensitive to chemotherapy will, in the majority of cases, eventually recur as a resistant tumor, which will then progress. Much of our understanding of drug resistance mechanisms comes from the study of tumor cell lines grown in tissue culture. We now understand...