CCL21 and rheumatoid arthritis: In RA, genome-wide association studies (GWAS) revealed numerous susceptibility alleles including HLADRB1, PTPN22, TRAF1/C5, STAT4, PADI4, IRF5, FCGR, IL2RA, IL2RB, CD40, CCR6, and CCL21. More than 40 single nucleotide polymorphisms (SNPs) have been associated with RA [18].