Juglone (Regianin) is a natural naphthoquinone found in the black walnut (J. nigra) and other plants in the Juglandaceae family. Juglone also irreversibly inhibits peptidyl-prolyl cis/trans isomerases of the parvulin family, including human Pin1, yeast Ess1/Ptf1, and E. coli parvulin (Ki = 55.9 nM). Juglone also blocks transcription by RNA polymerases I, II, and III (IC50s = 2-7 μM) and attenuates kidney fibrosis in rats treated with unilateral ureteral obstruction, both through Pin1-independent mechanisms.