EnSens® protease activity assay kits are based on a proprietary fluorescent molecular rotor dye, which does not fluoresce when free in solution. The other component of the EnSens® kits, are its substrates, which are recombinant (E. coli) proteins comprising two domains. The substrate’s activation domain binds the molecular rotor dye, while the inhibitory domain interacts with the activation domain, blocking dye binding. The two domains are connected by a flexible linker that includes a protease cleavage recognition site, which can be as long or short as needed to impart selectivity. This modular nature enables rapid development of novel and selective protease substrates. When cleavage by the target protease separates the two domains, the activation domain dimerizes and binds the dye, causing it to fluoresce brightly (Ex./Em. 635/656 nm).