Abstract: Objective 　To observe the effect of PIN1 antisense nucleotide on cell proliferation and cycle of human mammary cancer cell line MCF-7. Methods 　The eukaryotic vector named pPINas, expressing PIN1 antisense nucleotide, was const ructed. MCF-7 was transfected with pPINas and selected in the culture with G418. The selected clone MCF-7 PINas was checked for expression of PIN1 by RT-PCR and Western blot . The proliferation of cells and the change of cell cycle in clone MCF-7PINas were investigated by MTT and Flow Cytomet ry respectively. Results 　The expression of PIN1 at mRNA and protein level in MCF-7 PINas clone was down-regulated remarkably. MTT showed the proliferation of MCF-7PINas was retarded obviously cont rasting to MCF-7 ( P < 0. 05) . But the cell cycle distribution had no significant change ( P > 0. 05) . Conclusion 　Since blocking PIN1 expression with antisense nucleotide could depress the proliferation activity of MCF-7 remarkably, PIN1 may be a potential target of gene therapy for human mammary cancer.