Well, we don't have an explanation why it happened, but it happened. There is a mistake in the database of abbreviations… Luckily, it is easy to fix.

Please follow these steps: 1 Start Peptide Companion 2 Click on "Write from Scratch" 3 Select "Hcy" 4 If formula shown for Hcy is not C4H6NOS, then click on "Edit Abbreviation" 5 In the window "Abbreviation Editor" type formula C4H6NOS 6 Click OK 7 Answer "yes" to question "Overwrite existing abbreviation?" 8 Answer "No" to question "Add default atoms?" 9 Answer "Yes" to question "Is this unnatural amino acid?" 10 Exit program 11 Answer "Yes" to question "Should I write all changes …?" (You will need your password.)