Cyclin B is a central regulator of transition from the G(2) phase of the cell cycle to mitosis. In mammalian cells two B-type cyclins have been characterised, cyclin B1 and B2. Both are expressed with a maximum in G(2) and their synthesis is mainly regulated on the transcriptional level. We show that a single cell cycle genes homology region, lacking a functional cell cycle-dependent element in tandem with it, contributes most of the cell cycle-dependent transcription from the cyclin B1 promoter. The coactivator p300 binds to the cyclin B1 promoter and synergises with the transcription factor NF-Y in activating transcription of cyclin B1.