This enzyme‐responsive nanocarrier is tailored for the covalent cross‐linking aggregation of peptide‐modified upconversion nanocrystals (UCNs) at tumor sites.[75] Upon arrival at the tumor location after intravenous (i.v.)injection, the surface peptide chains of UCNs were cleaved by the tumor‐specifically overexpressed enzyme, cathepsin B (CtsB). This evidence concerns the gene CTSB and neoplasm.