The invention relates to a system for detecting the presence of a convertase cleaving activity, the system comprising a molecule comprising a proteolytic cleavage site with the amino acid sequence -R-X1-X2-R-, wherein R represents the amino acid arginine, X1 and X2 are selected independently among any amino acid, preferably from any one of the twenty two (22) standard amino acids wherein said system comprises a molecule, which comprises a first part and a second part separated by said cleavage site, and wherein said molecule, when cleaved at said cleavage site, loses or obtains a chemical, physical or biological property, including properties selected from fluorescence, catalytic and/or other biologic activity, said property depending on the close physical proximity or on the absence of such proximity, respectively, said first and second parts of the molecule that precede and follow the cleavage site, and which physical proximity or absence of proximity is thus lost or obtained, respectively, by cleavage through a proprotein convertase wherein said system further comprises at least one localising element, which directs the system, when present in a human or animal cell, to a specific cell compartment, to the extracellular compartment, or to the cell surface.