 Secondly, CuAAC outperforms SPAAC in click reactions where proteins with a high cytosine content, such as 4RepCT, are present.The SPAAC process, in the presence of proteins like 4RepCT, will often create ‘clicks’ in off-target sites resulting in the ligand conjugating to the wrong part of the protein and rendering the protein essentially useless.In order to maximize the number of functional sites along the fiber, CuAAC is preferred.