Approximately 67% of RA patients are ACPA-positive, establishing ACPAs as a crucial diagnostic marker for early, undifferentiated arthritis.40,41 The enzyme peptidyl arginine deiminase (PAD) catalyzes citrullination, a calcium-dependent process that converts positively charged arginine into neutral, polar citrulline. The gene discussed is PRTN3; the disease is rheumatoid arthritis.