Chemotherapy is the use of drugs to kill cancer cells, usually by stopping the cancer cells' ability to grow and divide. Chemotherapy sensitivity and resistance assays are tests done in a laboratory on the cells from part of the tumor removed during surgery or biopsy. These tests have been looked at in research studies as a way to predict whether chemotherapy will help treat the tumor or if the tumor is resistant to chemotherapy, meaning that the chemotherapy will not help treat the tumor.