Background and purpose: Estrogen receptor (ER)-positive breast cancer always presents a dilemma for resistance to endocrine therapy in a long time. The recent studies showed that the expression of estrogenresponsive finger protein (Efp) and polo-like kinase 3 (Plk3) had a close relationship with breast cancer development. This study was to explore the expression correlation between Efp and Plk3 in ER-positive brea

st cancer in order to understand the influence of Efp and Plk3 on the drug resistance. Methods: The expression of Efp and Plk3 in 74 cases of ER-positive breast cancer was detected by SP immunohistochemistry. The clinical significance was then analyzed. Real-time fluorescent quantitative polymerase chain reaction (RTFQ-PCR) and Western blot were used to detect the expression of Efp and Plk3 in ER-positive MCF-7 cells. Results: No significant relationship was found between Efp and Plk3 expression and the clinicopathological features of 74 cases of ER-positive breast cancer (P0.05). The number of cases whose Efp showed positive expression was 51 (68.9%)

while the number of cases whose Plk3 showed positive expression was 23 (31.1%). Chi-square test analysis showed the expression of Efp and Plk3 was negatively correlated in 74 cases of ER-positive breast cancer (χ

2

=8.837

P0.05). The result of RTFQ-PCR showed that the expression of Efp mRNA in MCF-7 cells was up-regulated by estrogen stimulation

whereas Plk3 mRNA was not changed. The result of Western blot showed that the expression of Efp protein in MCF-7 cells was increased by estrogen and MG132 stimulation

whereas Plk3 protein was decreased. Conclusion: The expression of the Efp protein is negatively correlated with Plk3 protein in ER-positive breast cancer. High expression of Efp may be involved in the resistance to endocrine therapy.