The various combinations of surface markers used to identify breast cancer stem cells include CD44+/CD24-/low, aldehyde dehydrogenase (ALDH1) activity (identified by the Aldefluor assay), and more recently CD44+/CD49f+/CD133/2+; all define subpopulations of cells that have been shown in limiting dilution xenograft transplantation assays to have increased tumorigenic potential[4-7]. The gene discussed is CD44; the disease is breast cancer.