An inhibitor of Peptidyl arginine deiminase 4 (PAD4) activity with an IC50 value of 21.6 μM in an in vitro activity assay. It irreversibly inactivates (kinact/KI = 3,000 M-1min-1) PAD4 by covalently m odifying an active site cysteine that is important for its catalytic activity. Dysregulated PAD4 activity has been implicated in cancer and rheumatoid arthritis. F-amidine also inhibits PAD1 and PAD3 with IC50 values of 29.5 and 350 μM, respectively. F-amidine is cytotoxic to HL-60, MCF7 and HT-29 cancer cell lines (IC50s = 0.5, 0.5 and 1 μM, respectively).