STEAP4 and rheumatoid arthritis: Previous studies indicated that IL-8 (a counterpart of mouse CXCL2) and IL-6 are induced by TNFα in FLS from RA patients26, 28, 29, and STEAP4 (human ortholog of TIARP) knock-down enhance the production of IL-8 and IL-6 by TNFα stimulation14.