Peptidyl arginine deiminase 4 (PADI4 or PAD4) is one of five enzymes in the PADI family. PADIs 1-4 catalyse the post-translational modification of peptidyl arginine residues to citrulline in a wide range of protein substrates. PADI4 is involved in cell signalling processes including apoptosis, differentiation, and regulation of transcription. Dysregulation of PADI4 is implicated in various diseases including rheumatoid arthritis, lupus and several cancers. PADI4 has a key active site Cys residue required for catalysis and there are known covalent small molecule binders. Fluoroamidine (1) is a small molecule inhibitor of PADI4; as an arginine mimetic (Figure ) it binds in the active site of PADI enzymes and covalently reacts with the active site cysteine (C645). However, it has limited selectivity for PADI4 over PADI1. From a screen of synthetic peptides containing 1, the tripeptide TDFA was identified with an IC50 of 2.3 µM and >15-fold selectivity for PADI4 over PADI1. Based on this precedent we envisaged that developing a high-throughput methodology to identify larger covalent peptides including the fluoroamidine warhead might provide a route to even more potent and selective inhibitors.