Cancer stem cell concepts--lesson from leukemia

Although monoclonal in origin, most tumors appear to contain heterogeneous populations of cancer cells. One possible explanation of this tumor heterogeneity is that human tumors are not merely monoclonal expansions of a single transformed cell, but rather caricatures of normal tissues, and their growth is sustained by cancer stem cells (CSCs). This hierarchy model, first developed for human myeloid leukemias, is supported by mounting evidences today. This conceptual shift has important implications, not only for understanding tumor biology but also for developing and evaluating effective anticancer therapies. We review a history of the development of cancer stem cell concepts in hematology and recent topics of leukemic stem cells (LSCs).