Objective:To establishMDA-MB-231 cell line stably expressing inducible small hairpin RNA（shRNA）targeting polo-like kinase 1（PLK1）

to explore the effects of PLK1 knockdown on cell proliferation and cell cycle in MDA-MB-231 cells as well as to explore its possible mechanism. Methods:Two oligonucleotides targeting PLK1 were synthesized and cloned into lentivirus expression plasmid p LVTHM according to si RNA principle（named PLK1 shRNA）. MDA-MB-231 cells were infected at first with lentivirus prepared by p LV-t TR/KRAB-Red empty vector

and then infected with another lentivirus prepared by PLK1 shRNA recombinant plasmid.MDA-MB-231 cells infected with aforementioned lentivirus were treated with doxycycline（DOX）for 96 hours

the expression of PLK1 m RNA and protein was determined by real time PCR（q RT-PCR）and Western blot respectively. MTT method was used to determine cell proliferation and flow cytometry and PI staining were used to detect cell cycle. Results :The lentivirus expression plasmids containing PLK1 shRNA were constructed successfully

and MDA-MB-231 cells were infected with the lentivirus prepared by the aforementioned plasmids. MDA-MB-231 cell line expressing stably inducible shRNA targeting PLK1 was established successfully and PLK1 expression levels were decreased obviously at m RNA and protein levels after induction by DOX（P<0.01）. PLK1 knockdown could obviously inhibit the proliferation of MDA-MB-231 cells and arrest the cell cycle in G2/M phase（P<0.05）. Conclusions:PLK1 knockdown induced by DOX could inhibit MDA-MB-231 cell proliferation and arrest cell cycle in G2/M phase

its possible mechanism may involve in ERK1/2/Fra1/ZEB1 signal pathway.