CD44 and cancer: The mechanism of homotypic aggregation is the result of multifactorial action and is closely related to tumor-specific antigens (e.g., Thomsen–Friedenreich antigen [29], carcinoembryonic antigen [32], and glycoprotein 100 [33]) and other adhesion protein ingredients (e.g., EpCAM, N-cadherin, E-cadherin, galectin-1, galectin-3, integrins, CD24, CD44, and CD47 [34–36]) derived from the surface of the cancer cell membrane.