Al-Hajj et al. initiated the study of a different subgroup of cells within human breast carcinomas, which could recreate new neoplasms in immunocompromised mice. These tumor-starting cells were determined from the nontumorigenic cell population as they showed the CD44+/CD24- maker phenotype, which was associated with cancer stem cell features [20,35]. The gene discussed is CD44; the disease is neoplasm.