Type 1 protein phosphatase (PP1), a serine/threonine phosphatase, is highly conserved in eukaryotic cells. Four isoforms of PP1 have been characterized: PP1alpha, PP1delta, PP1gamma1 and PP1gamma2 . Involvement in cell cycle regulation is one of the biological functions of PP1 . It has been illustrated that PP1 dephosphorylates Rb and cdc25 during mitosis . A cell cycle-dependent phosphorylation at Thr320 of PP1alpha by cdc2 kinase inhibits PP1alpha activity .