Review mophiles have only a fraction of the cysteine compared with mesophiles. An alternative view is that, as life thrived in a more temperate environment, new opportuni- ties have emerged for this uniquely reactive amino acid. Additionally, proteins have been exposed as being in- herently hydrolytically labile across Xaa–Cys sites in a manner that was not widely appreciated in the past. This may find application in proteomics as well as in the thio- ester syntheses that we have explored thus far. Major challenges to address in thioester formation from native peptide sequences are the requirements for elevated reaction temperatures and acidic pH, especially when several “device”-driven routes produce thioesters at ambient temperature and neutral pH. 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