Schematics of protein splicing, side reactions, trans -splicing, conditional protein splicing and expressed protein ligation. A. Protein splicing. B. N-terminal cleavage side reaction. C. C–terminal cleavage side reaction. D. Protein trans- splicing (PTS). E. Conditional protein splicing (CPS). The brown color of the intein box in E indicates an inactive intein, and the blue color is active. F. Expressed protein ligation (EPL). The black box can be a protein with an N-terminal Cys, either a synthetic peptide or a protein with an N-terminal Cys revealed by proteolysis or intein-mediated cleavage.