To verify our hypothesis, in the present study, we determined the role of Nr4a1 in renal fibrosis and the involvement of p38 MAPK in Nr4a1-mediated functions by using UUO mice and TGF-β1-treated HK-2 cells as in vivo and in vitro models of renal fibrosis. This evidence concerns the gene NR4A1 and renal fibrosis.