By CD44 and CD24 flow cytometry, a CD24−CD44med subpopulation (referred to as P1 thereafter) with positive but weak CD44 staining, and a CD24−CD44high subpopulation (referred to as P2 thereafter) with stronger CD44 staining were observed in breast tumors and the isogenic MCF10 breast cancer cell line series that include MCF10AT, MCF10CA1h, and MCF10CA1a46. Here, CD44 is linked to breast cancer.