CD44 and breast cancer: Cluster of differentiation 44 (CD44) is a single-chain transmembrane adhesion glycoprotein, which is involved in many signal pathways in physiological and pathological processes, especially the occurrence, progress and metastasis of tumors.62 CD44 is overexpressed on the surface of many tumors, including liver cancer, breast cancer, colon cancer, lymphoma, etc.63 It is considered as a marker of many cancer stem cells (CSCs),64 so it can be a cell surface target for active drug targeting and treatment.