CD44 and cancer: The first models of CSC were demonstrated using leukemia cells, in which CD34+/CD38- cell fractions alone were able to recapitulate disease morphology when transplanted into immunocompromised mice [21,22] In solid tumors, small populations of cancer stem cells have been defined by expression of cell-surface markers such as CD133 (in colon cancer) or CD44 (in breast cancer) and the ability to form tumors in immune-deficient mice [9,23-25].