Until the early 1990s,human cancers were considered a morphologically heterogeneous population of cells.In 1997,Bonnet et al[1] demonstrated that a small population of leukemia cells was able to differentiate in vivo into leukemic blasts,indicating that the leukemic clone was organized as a hierarchy;this was subsequently denoted as cancer Until the early 1990s, human cancers were considered a morphologically heterogeneous population of cells. In 1997, Bonnet et al [1] demonstrated that a small population of leukemia cells was able to differentiate in vivo into leukemic blasts, indicating that the leukemic clone was organized as a hierarchy; this was succeeded as as cancer