CD44 and cancer: HA-based drug nanocarriers are advanced because HA functions in the following aspects: (1) improving the biocompatibility and biodegradability of the drug delivery system, thereby enhancing efficacy and reducing toxicity; (2) enhancing the dispersity of nanocarriers in aqueous solution and improving the drug delivery; (3) constructing tumor-targeted drug delivery systems, as HA is a natural ligand to the endocytic HA receptor, CD44, which is overexpressed in many cancer cells.