CD44 and neoplasm: These include (1) cell surface markers such as CD133 and CD44; (2) phenotypic characteristics such as exclusion of Hoechst 33342 dye (side population) and aldehyde dehydrogenase 1 (ALDH1) activity; and (3) functional characteristics such as chemo- or radiotherapy resistance, growth as spheroids in serum-free medium, and tumor initiation in vivo (see Table 1).