MS raw data were analyzed using MaxQuant software, standard settings were used with the following changes and additions: The modi8ied FASTA databases with individual substitutions of cysteines with the placeholder "U" (selenocysteine) were used . MN551 covalently modi8ied cysteine (C(26)H( )N(3)O( )FPSe(-1)S) was set on the placeholder amino acid "U". In MaxQuant settings, acetylation (Protein-N terminus), oxidation of methionine (M), and MN551 modi8ied Cysteine (on placeholder U) are set as variable modi8ications. Carbamidomethyl (C2H3NO) was set as 8ixed modi8ication on cysteine. The digestion enzyme was set to Trypsin/P with maximum number of missed cleavages of 2. The relative intensity of peptide LDSIICVK without and with a modi8ied cysteine (placeholder "U") were analysed by normalising to total intensity of SOCS2 protein.