IDO1 and rheumatoid arthritis: The IDO expression level significantly increased in the inflammatory cytokine-treated H-SF-MSCs and RA-SF-MSCs, compared with the nontreated groups, when normalized with YWHAZ, the most stable RG (p < 0.0001 and p = 0.0002), PPIA (p < 0.0001 and p < 0.0001), and TBP (p < 0.0001 and p < 0.0001).