Pin1/Ess1p is a highly conserved WW domain-containing peptidyl-prolyl isomerase (PPIase); its WW domain binds specifically to phospho-Ser/Thr-Pro sequences and its catalytic domain isomerizes phospho-Ser/Thr-Pro bonds. Pin1 PPIase activity can alter protein conformation in a phosphorylation-dependent manner and/or promote protein dephosphorylation. Human Pin1 interacts with mitotic phosphoproteins, such as NIMA, Cdc25 and Wee1, and inhibits G(2)/M progression in Xenopus extracts. Depletion of Pin1 in HeLa cells and deletion of ESS1 in S. cerevisiae result in mitotic arrest. In addition, Pin1/Ess1p play roles in transcription in S. cerevisiae and in mammalian somatic cells. The S. pombe genome sequence has an open reading frame (ORF) that has 47% identity with Pin1. Expression of this ORF rescued the growth defect caused by ess1 deletion in S. cerevisiae, indicating that S. pombe Pin1p is a functional Pin1 homologue. Overexpression of pin1(+) in S. pombe caused slow growth and a G(1) delay. Deletion of pin1(+) (pin1 Delta) did not affect cell cycle progression or cell growth, but increased sensitivity to the cyclophilin inhibitor, cyclosporin A, suggesting that cyclophilin family PPIases have overlapping functions with the Pin1p PPIase. Deletion of pin1(+) did not affect the DNA replication checkpoint, but conferred a modest increase in UV sensitivity. Furthermore, the pin1 Delta allele caused a synthetic growth defect when combined with either cdc25-22 or wee1-50 but not the cdc24-1 temperature-sensitive mutant. The pin1 Delta strain showed increased sensitivity to the PP1/PP2A family phosphatase inhibitor, okadaic acid, suggesting that Pin1p plays a role in protein dephosphorylation as a result of its ability to increase the population of phospho-Ser/Thr-Pro peptide bonds in the trans conformation that is required for PP2A-mediated dephosphorylation. Our genetic data also suggest that Pin1p might function as a positive regulator of Cdc25p and Wee1p.