Protein-arginine deiminase type-4, also known as HL-60 PAD, Peptidylarginine deiminase IV, Protein-arginine deiminase type I V and PADI4, is a cytoplasm and nucleus protein which belongs to the protein arginine deiminase family. PADI4 is expressed in CD34+ stem cells in normal tissues, and many more CD34+ cells expressing PADI4 are present in tumour tissues. PADI4 post-translationally converts peptidylarginine to citrulline, a process called citrullination. Studies have demonstrated the high expression of PADI4 in various malignant tumour tissues. PADI4 is also expressed at high levels in the blood of patients with some malignant tumours. Citrullination of histone, cytokeratin, antithrombin and fibronectin have been confirmed to be involved in abnormal apoptosis, high coagulation, and disordered cell proliferation and differentiation, all of which are main features of malignant tumours. PADI4 may play an important role in tumourigenesis. Genetic variations in PADI4 are a cause of susceptibility to rheumatoid arthritis (RA). It is a systemic inflammatory disease with autoimmune features and a complex genetic component. It primarily affects the joints and is characterized by inflammatory changes in the synovial membranes and articular structures, widespread fibrinoid degeneration of the collagen fibers in mesenchymal tissues, and by atrophy and rarefaction of bony structures.