Of the top 30 most enriched sequences, few contained warhead 2. Those that did, were also found in previous selections and did not significantly increase in abundance over the course of the selection, suggesting that they are contaminations that bind C645 (Figure ). The other sequences most frequently enriched had lost the CGSGSGS linker or contained a very large proportion of Cys residues. There was also a low number of sequences for the later rounds (Figure ). These factors all indicated that even through presentation on a tight binding cyclic peptide, warhead 2 could not be forced to react at alternative Cys residues in PADI4, hence 2 requires a specifically activated, nucleophilic Cys, like the active site C645.