This peptide is a second complement component (C2), the physiological substrate for the proenzyme Cls, first complement component. The C1 component is a complex of the binding protein C1q and two proenzymes, C1r and C1s. The C1s protease of the C1 complex initiates the host defense pathway cleaving the C4 and C2 components to instigate a sequence of activation steps of other components of the complement system, culminating in the formation of the membrane attack complex, which induces cell lysis. This peptide employs DABCYL / EDANS as FRET pair.