Fig. 1. DEC is a new reduction sensitive linker that can modify aliphatic amines and release them in a traceless manner. (a) DEC is composed of two rapidly hydrolyzing self-immolative linkers connected in series, in particular a thiol-ethyl carbonate linker and a 1,6-elimination linker. In the presence of reducing reagents such as GSH, the disulfide bond in DEC is reduced, generating a free thiol which spontaneously cyclizes into a cyclic thioether, causing the release of its phenol (step 1). The phenol intermediate triggers the 1,6-elimination process to release the aliphatic amine (step 2). (b) DEC has the potential to accelerate the development of new prodrugs and protein delivery vehicles. DEC was able to PEGylate Cas9 protein, which was able to diffuse through brain tissue significantly better than free Cas9. In addition, DEC was also used to synthesize a self-delivering Cas9 RNP composed of Cas9 RNP conjugated to the CPP (Arg)₁₀. Finally, conjugation of donor DNA to Cas9 with DEC–PEG dramatically increased the homology directed repair (HDR) rate of the Cas9 RNP.