CD9 and cancer: Furthermore, several other exosome membrane proteins, such as the transmembrane protein platelet-derived growth factor receptor (PDGFR), the tetraspanin superfamily members, CD63, CD9, and CD81 (with their two extracellular loops), and lactadherin (with its C1C2 domain) can also be engineered to confer cancer cell targeting specificity [155, 156].