PADI4 and rheumatoid arthritis: PAD4-mediated protein citrullination is involved in the regulation of various physiological processes, and dysregulated citrullination has been shown to be associated with many autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis, sepsis, Corona Virus Disease 2019 (COVID-19), cancer and thrombosis [22,23,24,25,26,27].