CD44 and breast cancer: CD44 is a transmembrane glycoprotein widely expressed in physiological and pathological systems,22 and is associated with tumor metastasis and BCSCs.4,5,50 According to recent studies, the cleaved intracellular domain of CD44 (CD44ICD) activates stemness factors such as Nanog, Sox2, and Oct4, and contributes to tumorigenesis of breast cancer, which may explain the effect of CD44 on BCSC maintenance.51 In this study, we showed that BMP-2 upregulated CD44 expression and promoted CSC development in breast cancer cells.