A maternal Tubulin promoter and sqh 3'UTR regulate expression of a fluorescent biosensor of Cdk2 activity. The biosensor consists of amino acid residues of 994-1087 of Hsap\HELB (this fragment contains a nuclear localization signal, Cdk phosphorylation sites, and a nuclear export signal), tagged at the C-terminal end with mCherry. In the absence of Cdk activity, the biosensor protein is localized to the nucleus. After phosphorylation by an active Cdk/Cyclin complex, the biosensor protein is excluded from the nucleus. The ratio of cytoplasmic-to-nuclear sensor protein thus provides a readout of Cdk-mediated phosphorylation.