Table 1 presents a summary of the currently known CSC biomarkers and their characteristics. CSCs can, however, also be found in single CSC marker–negative cell populations, which means that any single CSC marker does not cover all CSC populations. Therefore, multiple CSC markers are needed to encompass most CSC subsets. For example, CD44+/CD24−/ALDH1+ breast cancer cells represent the most enriched set of breast cancer stem cells (BCSCs). This method of combining multiple CSC markers to define CSC populations is thus highly predictive of tumor malignancy (56). This evidence concerns the gene ALDH1A1 and breast cancer.