CD9 and breast cancer: These EVs are produced by human breast cancer SUM159 cells stably transfected with the two components of the split protein system (DSP1-coupled to the tetraspanin CD9 for optimal expression at the EV membrane, and DSP2) and permits the discrimination of EV uptake by recipient cells from EV binding to the cell surface thanks to the use of EndurenTM (Promega Biotech, Alcobendas, Spain), a cytopermeable chemically protected Renilla luciferase substrate that only becomes the real substrate after the action of intracellular sterases [47].