Cancer Stem Cells (CSCs) are a small subset of cells within a tumor which posses intrinsic stem cell qualities of self-renewal and differentiation. They have the capacity to expand the cancer stem cell pool as well as differentiate into the heterogeneous non-tumorigenic cancer cells that make up the bulk of the tumor. A number of proteins have been used to identify CSCs, including CD44, CD133, ALDH1, ABCG2, CD24, BMI1, CD90, Nestin, CD166, and others. Antibodies to these markers may prove useful in investigating CSCs. However, it is important to note that many markers are not unique to cancer stem cells, and are also expressed in normal stem cells. Be sure to consult the latest literature, as the cancer stem cell model continually evolves with the emergence of new findings.