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 Dl protein was performed in 66 patients treated with tamoxifen for more than 2 years. Patient survival and correlation between cyclin D1 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 Dl expression 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 Dl expression and S-phase. Patients with tumors expressing cyc1in Dl 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. Cyclin D1 may have a role other than cell cycle regulator in ER positive breast cancer, such as differentiation signal.