In summary, BANK1 contributes to the RA genetic susceptibility background, but the association may depend on the BLK rs13277113 genotype, in that our results suggest an epistasis between BANK1 and BLK. This epistasis could explain the discrepancies in the various reported association studies and suggest that BANK1 rs3733197 status should be re-analyzed for BLK rs13277113 status. The gene discussed is BLK; the disease is rheumatoid arthritis.