CTSL and cancer: Dr. Chen’s group used the high metabolic activity of cancer-overexpressed enzymes (histone deacetylase and cathepsin L-responsive acetylated azidomannosamine) to design an enzymatically activatable Ac4ManAz analog, which was subjected to a metabolic labeling process to form azido-containing sialic acid, and further conjugated this analog to glycoproteins highly expressed on the cancer cell surface [68].