Visualising cell-cycle progression in living embryos is essential for improving our understanding of developmental processes. The fluorescent ubiquitylation-based cell-cycle indicator (Fucci), which was generated by fusing the ubiquitylation domains of Cdt1 and Geminin to different fluorescent proteins, has been used to label G1 and S/G2/M phase nuclei orange and green, respectively. Cell cycle dynamics during development have been studied in transgenic mice expressing Fucci under the control of the CAG promoter but Fucci expression levels are somewhat variable. Now, Shinichi Aizawa and colleagues () describe two new mouse cell-cycle reporter lines that use Fucci2, a Fucci derivative that emits red and green fluorescence. The R26p-Fucci2 transgenic line uses the Rosa26 promoter and harbours the G1 and S/G2/M phase probes in a single transgene to ensure their co-inheritance. In the R26R-Fucci2 transgenic line, the two probes are incorporated into the Rosa26 locus conditionally to allow cell-cycle analysis in specific cell types. Time-lapse imaging experiments suggest that both reporter lines hold great promise for studying cell-cycle behaviour in vivo.
