F-amidine is an inhibitor of protein arginine deiminases (PADs) that is selective for PAD1 and PAD4 (ICs = 29.5, 350, and 21.6 µM for PAD1, PAD3, and PAD4 in vitro, respectively). It irreversibly inactivates all four PAD subtypes (k/K = 2,800, 380, 170, and 3,000 Mmin by covalently modifying an active site cysteine that is important for its catalytic activity. F-amidine is cytotoxic to HL-60, MCF-7, and HT-29 cancer cell lines (ICs = 0.5, 0.5 and 1 μM, respectively).