To identify proteins potentially interacting with PAD4, we first used a mouse monoclonal antibody clone OTI4H5 (Abcam) to immunoprecipitate PAD4 from lysates of neutrophils from three different patients with RA and one healthy donor and then subjected the precipitated proteins to LC-MS/MS mass spectrometry using an Orbitrap instrument. The gene discussed is PADI4; the disease is rheumatoid arthritis.