APEX1 and rheumatoid arthritis: Furthermore, the RA patient group had a significantly higher APE1/Ref-1 level in their SF than that of the OA patient group (0.68 ± 0.31 ng/mL for RA group vs. 0.31 ± 0.12 ng/mL for OA group, p < 0.001) (Figure 1b).