Upon entering into the tumor microenvironment, the peptide linker was cleaved by the highly expressed extracellular MMP2, leading to the detachment of PEG chains and the exposure of TATp to promote the internalization of nanoparticles into cancer cells.488,489 In another study, a short PEG chain was covalently linked to the tumor-targeting motif RGD through an MMP-2 substrate peptide (PLGVR) to block its activity in the circulation. Here, MMP2 is linked to cancer.