Although the consensus is that no single marker adequately defines CSC, a considerable amount of data indicates that tumor cells which express elevated levels of aldehyde dehydrogenase (ALDH) activity, ALDHpositive cells, have been shown to have CSC-like properties, in particular, the ability to initiate tumor growth at low numbers in immunodeficient mice [6–12]. The gene discussed is LDHA; the disease is neoplasm.