There are five isoforms of human peptidylarginine deiminases (PADs) that catalyze citrullination, but primarily PAD2 and PAD4 have been implicated in RA due to their presence in immune cells (2) and RA synovial tissue (3), their contributions to murine inflammatory arthritis (4, 5), and the presence of RA-associated single nucleotide polymorphisms in both PADI2 and PADI4 (6, 7). Here, PADI4 is linked to rheumatoid arthritis.