PTPN22 and rheumatoid arthritis: After checking that PTPN22 rs2488457, rs2476601 and rs33996649 were in HWE, we observed a significant down-regulation of PTPN22 in patients with RA carrying PTPN22 rs2488457 risk allele G and rs2476601 risk allele A compared to healthy controls (p = 0.004 and 0.007, respectively) (Fig. 2a,b and Table 1).