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Studying the real-time kinetics of gene expression quantitatively in vivo is technically very challenging but could offer genuine insight into fundamental biological processes. In two related papers in the Journal of Cell Biology and EMBO Journal, Ono and colleagues describe the novel reporter mouse Tocky (‘timer of cell kinetics and activity’; the homonym ‘toki’ means ‘time’ in Japanese). The Tocky model uses a fluorescent timer protein that spontaneously shifts its emission from blue to red with a distinct half-life. Through appropriate pairing of the timer protein and quantitative modeling, it is potentially possible to monitor transcriptional activity in vivo for any gene of interest. The authors use Tocky mice to investigate the kinetics of gene expression in regulatory T cells during differentiation and under inflammatory conditions.