Protein arginine deiminase 4 (PAD4) mediates the transformation of protein arginine into citrulline. Citrullination of proteins has normal roles in gene regulation and pathological roles in immunological and inflammatory diseases. GSK199 is a reversible inhibitor of PAD4 (IC50 = 200 nM) that binds to the low-calcium form of the enzyme and is selective for PAD4 over PAD1-3.It is less potent than the related PAD4 inhibitor GSK484 , which demonstrates an IC50 value of 50 nM. GSK199 can inhibit the citrullination of PAD4 target proteins and diminish the formation of neutrophil extracellular traps in mouse neutrophils.