HLA-DRB1 and rheumatoid arthritis: In view of the associations between SNPs in PADI4 and anti-CCP-positive RA and the known interaction between the SE and smoking as risk factors for anti-CCP-positive RA, we analyzed the interactions between PADI4 SNPs and other risk factors, i.e., HLA-DRB1*04, HLA-DRB1*01, PTPN22 R620W, and smoking on the risk of anti-CCP-positive or anti-CCP-negative RA.