CD44 and cancer: Since most of the inorganic nanoparticles are not naturally tumor-specific, several methods have been used to enhance the tumor-homing capability of nanoparticles such as conjugation with antibody targeting the GD2 antigens overexpressed on the surface of solid tumor cells [136], conjugation with an antibody targeting the cancer-specific antigen epidermal growth factor receptor [138], or conjugation with hyaluronic acid that could target CD44 on colon cancer cells [139].