Cancer research focuses on stem cells present in malignant tumors. Researchers believe current cancer treatments sometimes fail because they don't destroy the cancer stem cells. Think of cancer as a weed: the stem cells are the root while the remaining majority of the cells are the part of the weed above ground. If you remove only the leaves but not the root, the weed will grow back. The same is true for cancer: if you do not kill the cancer stem cells, the cancer is likely to return.