To elucidate the fibrosis mechanisms observed in 2/3 Nx + UUO mice, we evaluated the effect of indoxyl sulfate (IS), a typical uremic toxin accumulated in CKD, and transforming growth factor‐β (TGF‐β), a fibrosis‐related factor, on fibrosis using human hepatoma (HepG2) and RAW264.7 cells. The gene discussed is TGFB1; the disease is chronic kidney disease.