Juglone is a natural naphthoquinone found in the black walnut (J. nigra) and other plants in the Juglandaceae family. It has allelopathic actions, suppressing growth, photosynthesis, and respiration in plants and other organisms, although some bacteria can metabolize juglone. 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.