Abstract: A method of screening for a compound that affects mammalian cell cycle regulatory proteins, comprising (A) administering a compound to a cell line, wherein the cell line comprises genetic information comprising (1) a reporter gene operably linked to a gene expression control sequence, wherein the gene expression control sequence comprises an upstream activation sequence and a promoter, and the upstream activation sequence comprises a DNA region that binds to a transcription control factor that is regulated through phosphorylation by a cyclin/CDK phosphorylation system; and (2) a hybrid gene comprising a first coding region from a gene native to the cell line and a second coding region from a second gene, wherein the first gene encodes a gene product that affects phosphorylation by the cyclin/CDK phosphorylation system, and the second gene is mammalian and is homologous to the native gene, and the hybrid gene provides a gene product effective to permit normal cyclin/CDK regulation of the transcription control