The common polycationic nature of the endocytosed species suggests a general mechanism of cell entry. Among ubiquitous HSPGs that may act as receptors, syndecans are transmembrane proteins which may cluster to form focal points at the plasma membrane following their individual electrostatic binding to a polycationic particle. There is indeed strong evidence [15] that syndecan clustering induces their binding to the actin cytoskeleton and possibly the formation of tension fibers. Here tension, however, would not provide mechanical stability, but rather the energy to engulf the cationic particle and form an intracellular vacuole (Fig. 1).