Juglone (Regianin, CAS 481-39-0) is a natural compound that potently inhibits Pin1 and other parvulin-family PPIases (Ki = 55.9 nM). It also blocks eukaryotic RNA polymerases (IC50 2-7 µM) and has demonstrated in vivo activity in attenuating kidney fibrosis in rodent models, acting through both Pin1-dependent and independent mechanisms.