Evidence indicates that overexpression of cyclin D1 is an important event in malignant transformation of breast cancer cells. Therefore, cyclin D1 is a potential target for mechanistically-based chemoprevention/treatment of breast cancer. Treatment of serum-stimulated quiescent MCF-7 breast cancer cells with cyclopentenone (2-cyclopenten-1-one) blocked progression through G1 and into S phase. Growth arrest of the cyclopentenone-treated cells in G1 was associated with changes in the levels of several proteins that control the cell cycle, including a dramatic decrease in cyclin D1 protein expression. Cyclopentenone also decreased the abundance of cyclin D1 mRNA and nuclear transcripts, indicating that it regulated cyclin D1 expression at the transcriptional level. Cyclopentenone selectively inhibited the activity of the cyclin D1 and cyclin A promoters but not the activity of several other control promoters. Deletion analysis indicated that the cyclopentenone response element was located in the cyclin D1 core promoter. Additional functional studies showed that a sequence within the core promoter (CycY, located downstream from the initiator element) played an important role in activation of the cyclin D1 promoter in MCF-7 cells. Electrophoretic mobility shift assays demonstrated specific binding of the transcription factor BTEB to the CycY site. The cyclopentenone response element did not correspond to the CycY site but rather mapped to the initiator element itself. The overall results suggest that cyclopentenone interferes with the transcription initiation complex that assembles over the cyclin D1 initiator element, leading to selective inhibition of cyclin D1 gene transcription.