Cancers, when produce clinical signs have passed a long journey. There should be more than near 109 cells for a cancer to be clinically detectable. In order to achieve an acceptable remission, cell mass should be brought to less than 109 by different therapeutic methods. So even after remission significant amount of cancer cells are still present in the body and different methodologies with varying sensitivity and specificity have been used to monitor the remained small amount of residual cancer cells. They are usually monitored by searching for cells in small population having the same phenotypic or genotypic features of the original tumor. Cancer stem cells also are resistant to therapy and are among these cells.