A maternal Tubulin promoter drives expression of a fluorescent sensor that can be used to monitor the dynamics of Chk1 activity. The sqh 3'UTR is also present downstream of the coding sequence. The sensor consists of a amino acids 183-251 of the Hsap\CDC25C gene (this fragment contains a nuclear localization signal, a Chk1 phosphorylation site (S216), and a nuclear export signal), fused to EGFP. Phosphorylation of the S216 site triggers binding of 14-3-3, which masks the nuclear localization signal sequence, decreasing the nuclear import rate. The cytoplasmic-to-nuclear ratio of the fluorescent sensor thus provides a readout of Chk1 activity.