Pin1, also known as peptidyl-prolyl cis-trans isomerase NIMA-interacting1, is nuclear PPIase containing a WW protein interaction domain, and is structurally and functionally related to Ess1/Ptf1, an essential protein in budding yeast. Pin1 is thus an essential PPIase that regulates mitosis presumably by interacting with NIMA and attenuating its mitosis-promoting activity. Substrates of Pin1 include the mitotic regulators (Cdc25 phosphatase and NIMA, PLK I, Wee, and Myt1 kinases), several transcription factors like ?-Catenin, c-Jun, and the tumor suppressor protein p53 , and some specific proteins like the RNA Pol II, the cytoskeleton protein tau, and the G1/S protein Cyclin D1. Recombinant mouse PIN1 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.