The unique prolyl isomerase, Pin1, has been found to regulate mitosis through a simple conformational change (1) the cis-trans isomerization of phosphorylated Ser/Thr-Pro amide bonds in a variety of key cell cycle regulatory phosphoproteins, including the Cdc25 phosphatase, the p53 oncogene, and the c-Myc oncogene (1-3). Importantly, Pin1-catalyzed post-phosphorylation conformational changes can have profound effects on many key proteins in diverse cellular processes (4). Cells depleted of Pin1 are characterized by premature entry into mitosis, followed by mitotic arrest, nuclear fragmentation, and apoptosis, while overexpression of Pin1 inhibits the G2 to M transition (2, 5). Hence, Pin1 acts as a negative regulator for mitotic activity in G2, preventing lethal premature entry into mitosis. Pin1 is present at higher concentrations during mitosis (6), making it a potential target in the continuously dividing cells of cancer. The central role Pin1 plays in the cell cycle makes it an interesting target for inhibition, both for potential anticancer activity and for elucidation of the mechanism of mitosis.