Cyclin D1 expression is closely related with ER in breast cancer, We conducted this study to evaluate whether therapeutic response to tamoxifen is varied with levels of cyclin D1 expression in ER positive breast cancer patients.

Immunohistochemical assay for cyclin D1 protein was performed in 66 patients treated with tamoxifen for more than 2 years. Patient survival and correlation between cyclin Dl expression and biologic data of the patients were analyzed.

Cyclin Dl expression was detected in 46 (69.7%) and significantly reduced in poorly differentiated cancer (p=0.023). Cyclin D1expression was high in the tumors expressing Myc (15/15 vs 31/51; p=0.002), and was markedly increased in the tumors in which p27Kipl expression was repressed (30/38, 78.9%). However, the difference was not statistically significant (p=0.051). There was no significant relationship between cyclin D1 expression and S-phase. Patients with tumors expressing cyclin D1 showed better disease free survival and overall survival but the difference was not statistically significant.

Cyclin D1 expression was associated with cell differentiation but not useful in discriminating high risk group with tamoxifen treatment. Cyc1in D1 may have a role in process other than cell cycle regulator in ER positive breast cancer, such as differentiation signal.