PIK3CA and breast cancer: CD44 is a common marker of breast cancer stemness, which is capable of promoting a variety of functions independently or in cooperation with other cell-surface receptors via activation of diverse signaling pathways, including Rho GTPases, Ras-MAPK, and PI3K/AKT pathways, to regulate cell adhesion, migration, survival, invasion, and epithelial-mesenchymal transition (EMT) [68,69].