Identification of cancer stem cells: from leukemia to solid cancers

Cancer stem cells (CSCs) are widely considered to be a small cell population in leukemia and many solid cancers with the properties including self-renewal and differentiation to non-tumorigenic cancer cells. Identification and isolation of CSCs significantly depend on the special surface markers of CSCs. Aberrant gene expression and signal transduction contribute to malignancies of CSCs, which result in cancer initiation, progression and recurrence. The inefficient therapy of cancers is mainly attributed to the failure of elimination of the malignant CSCs. However, CSCs have not been detected in all cancers and hierarchical organization of tumors might challenge cancer stem cell models. Additionally, opinions about the validity of the CSC hypothesis, the biological properties of CSCs, and the relevance of CSCs to cancer therapy differ widely. In this review, we discuss the debate of cancer stem cell model, the parameters by which CSCs can or cannot be defined, and the advances in the therapy of CSCs.