CD44 and breast carcinoma: Furthermore, a subpopulation of CD44+CD49highCD133/2high cells isolated from ER-negative tumors was demonstrated to be enriched for xenograft-initiating cells capable of giving rise to triple negative and ER-negative/HER2-positive tumors [17], endorsing CD133 as a suitable molecule for the identification of CSCs in the most aggressive subtypes of breast cancer.