CD44 and neoplasm: This has led to the development of methods to enrich for subpopulations of cells with increased tumorigenic potential using either surface markers such as CD133 [9] or CD44 [21]; functional activities such as alhedyde dehydrogenase (ALDH) staining or Hoechst 33342 exclusion [22], [23]; or also in vitro culture systems in suspension as spheroids in well defined medium conditions [24], followed by the demonstration that engraftment of small amounts of cells in recipient mice is able to regenerate a complete tumor tissue resembling histologically and functionally the originating human tumor.