PADI4 and rheumatoid arthritis: Collectively, all of these factors initiate the early development of RA, including post-translational modifications of a wide range of cellular (collagen) and nuclear (histones) proteins, such as the conversion of the amino acid arginine to citrulline (a process called citrullination) through peptidyl arginine deiminase, a type IV enzyme (PADI4; EC: 3.5.3.15) [1,159].