D-type cyclins are implicated in cell cycle regulation. The cyclin D1 gene is amplified in 15-20% of breast cancers while the protein is overexpressed in 50% of these cancers (see Additional information ). Overexpression of cyclin D1 has been linked with oestrogen receptor (ER)a, because it can stimulate transcriptional functions in the absence of oestrogen (see Additional information ). A negative association in normal breast has been reported for ERa and Ki67 (a proliferation marker), although this relationship changes in many precancerous and ER-positive cancers. Therefore, the aim of the study was to investigate the interaction between cyclin D1, ER and Ki67 in normal and malignant breast material.