PADI4 and Arthritis: Hence, CRP is an acute-phase protein associated with inflammation, OST, a bone-derived protein that particularly plays a key role in bone remodeling processes related to arthritis, and PADI4, an enzyme involved in the posttranslational modification of proteins by citrullination, has been linked to autoimmune responses in arthritis, as citrullinated proteins are often targeted by the immune system in rheumatoid arthritis.