While being treated for cancer, people are assessed to see how the cancer is responding to therapy. The most successful treatment produces a cure. The least successful treatment produces no response. The interval from complete response to the time of death is the total survival time. In people who have a partial response, the duration of response is measured from the time of the partial response to the time when the cancer begins to enlarge or spread again.
Some cancers respond well to chemotherapy.
Others improve but aren't cured. Because multidrug-resistant genes are found in both normal cells and cancer cells, exposure to one drug can make a tumor resistant to unrelated cancer drugs. Presumably these genes exist to provide cells with the means to escape destruction by a noxious material. As a result, the cell may pump out the drug in self-defense, making the therapy ineffective. Researchers are trying to determine how to suppress the activity of these genes.