= Solution
<Fucci> uses two fluorescent <protein> fusions carrying cell-cycle-regulated degradation signals: a red/orange <Cdt1> fragment accumulates mainly in <G1 phase>, whereas a green <geminin> fragment accumulates through <S phase>, <G2 phase> and much of <M phase>. Opposing activities of <SCF ubiquitin ligase> and <anaphase-promoting complex> regulate reporter degradation through the <ubiquitin-proteasome system>; the fluorophores report those regulated stabilities rather than simply measuring total endogenous <Cdt1> and <geminin>.<Visualizing spatiotemporal dynamics of multicellular cell-cycle progression by Asako Sakaue-Sawano and others>[Abstract and reporter-design experiments]{ref}
\b[Live imaging follows individual <biological cells> through red, overlap and green states], allowing measurements of phase timing, spatial patterns and responses to perturbation without repeatedly fixing the sample. <Flow cytometry> can separate populations by reporter color. The basic two-color version does not independently resolve every phase, and a color transition is an indirect readout of <ubiquitin ligase> activity. Validate timing with <DNA> content or a <DNA replication> marker and control for altered reporter expression, fluorophore maturation and drug effects on degradation.
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