It is well known that sensitivity to anticancer drugs differs between cancers even if they show the same histological findings. CRA is used to remove potentially ineffective chemotherapy regimens to avoid the toxicity of potentially ineffective drugs. It may be crucial to select the best drug for the first course of chemotherapy, because anticancer drugs considerably increase the mutation rate in tumor cells and a significant shift toward resistance in patient who had received prior chemotherapy. Our assay technology is being developed, using four basic steps: 1) isolation of tumor cells from cancer patient, 2) incubation of cells with anticancer drugs, 3) assessment of cell survival, and 4) interpretation of the result.