GSK-199 is a potent, reversible inhibitor of PAD4 (IC₅₀ = 200 nM. It 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 (IC₅₀ = 50 nM). GSK-199 can inhibit the citrullination of PAD4 target proteins and diminish the formation of neutrophil extracellular traps in mouse neutrophils.