CTSB and adenocarcinoma: Besides MMPs, a cathepsin B cleavable sequence, PGFK, has been used to connect a polyanionic inhibitor to a Tat derived CPP that was conjugated to mesoporus silica quantum dot nanocarriers loaded with DOX (Table 2, entry 3).27,28 In the presence of endogenous cathepsin B levels, DOX was released and transported selectively to the nuclei of human adenocarcinoma cells while uptake was drastically decreased in the absence of cathepsin B.