Illustration of chemical steps during in vitro processing of IgG-intein fusion protein. Above the arrow, the chemistry of GyrA intein is shown. After intein-catalyzed N-S acyl shift, GyrA is substituted by Mesna. Mesna-thioester is cleaved by the probe functionalized with an N-terminal cysteine residue, and finally rearranges to a stable peptide bond. Below the arrow, the chemistry of DnaE intein is shown. The intein has to be activated by association of its N- and C-terminal part. The probe is attached to the C-terminal part and transferred after the intein-catalyzed N-S acyl shift.