ScienCentral is running a gentle introduction to cancer stem cells and their significance: "cancer stem cells were first discovered in 1994. Since then, cancer stem cells have also been found in breast cancer, brain cancers, and prostate cancer. ... in addition to the published cancers, I know of four other types of cancer where the stem cell population has been identified ... Using a sophisticated instrument called a flow cytometer that sorts out stem cells from tissue samples, the researchers can also isolate cancer stem cells from tumors. Then they can test the cells' ability to form new tumors. ... What we found was as few as 100 of the cancer stem cells could easily re-form a tumor whereas tens of thousands of the other cancer cells were totally unable to form a new tumor." Researchers believe that attacking these stem cells with targeted therapies will be an effective new direction for cancer research.