Inteins are intervening sequences that are post-translationally excised from the surrounding extein host protein.The mechanism involves 4 separate nucleophilic displacement reactions.Three splice junction residues act as nucleophiles,while other splice junction and internal residues facilitate these reactions.In standard inteins,splice junction motifs include Ser or Cys at the intein N-terminus,His-Asn at the intein C-terminus and Ser,Thr or Cys at the beginning of the C-extein.Step 1:the N-terminal splice junction is activated by an acyl rearrangement to form a (thio) ester.