Various investigators have isolated CSCs, which are proposed to be therapy resistant [48], [49] and responsible for relapse of the tumor following remission after therapy [50] as well as metastases [51]–[53], employing different phenotypes including side population (SP), CD44highCD24low/neg, ALDH1+, CD34high, CD133high, and CD49fhigh in either murine or human breast cancers [5], [22], [23], [54]–[58]. The gene discussed is ALDH1A1; the disease is breast cancer.