The present invention relates to the finding that cyclin D1 interacts in a ligand-independent fashion with coactivators of the SRC-1 family. The direct interaction of cyclin D1 enhances estrogen receptor (ER) mediated transcription and provides a novel target for the development of assays for substances which modulate the cell cycle. The invention provides assay methods for the prevention of growth of tumors, for assays for compounds useful in the prevention of tumors and compounds obtainable by such assays.