Immunohistochemistry demonstrated that only a small minority (∼1%) of tumor cells expressed a high level of ALDH, even in tumors that arose from implantation of purified ALDHhigh cells (Figure 9C), supporting the hypothesis that these represent true cancer stem cells, able to generate serially-transplantable tumors that contain mostly ALDHlow non-stem cells. The gene discussed is LDHA; the disease is cancer.