Given all our identified peptides bound to the active site cysteine of PADI4, in parallel we performed a selection on biotinylated PADI4 C645A (Figure ), to see if we could promote identification of peptide binders at alternative cysteine residues in PADI4 when the favoured C645 was not available. We chose the same selection conditions as the initial selection to increase our probability of identifying even poor binders. However, there was very minimal enrichment of positive recovery with the PADI4 C645A-bound beads, which was always far exceeded by the negative recovery with biotinylated streptavidin beads (Figure ).