CD44 and breast cancer: Subsequently, nanoemulsion effects on P-gp inhibition and paclitaxel cytotoxicity were studied in 2D and 3D breast cancer models using MCF-7 and MDA-MB-231 cells, which were selected for their distinct expression of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and CD44.