NRP1 and cancer: On the one side, the RGD sequence is known to be able to target tumor endothelium via interaction with ανβ3-integrin (a receptor overexpressed in tumor vasculature [21]); while on the other side, the RGDK sequence has been shown to bind the neutropilin-1 (Nrp-1, a transmembrane hub receptor with multiple ligands that is abundant on the surface of cancer cells [22]), and in so doing, promoted cell uptake and penetration.