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Many proteins and pathways of pharmaceutical interest impinge on ubiquitin ligases or their substrates. The cyclin-dependent kinase (Cdk) inhibitor p27, for example, is polyubiquitylated in a cell cycle–dependent manner by a ubiquitin ligase complex containing the F-box protein Skp2. Regulated turnover of p27 is due, at least partly, to its phosphorylation by Cdk2 on threonine 187, which generates a Skp2-binding site. We made a p27-luciferase (p27Luc) fusion protein and show here that its abundance, like that of p27, is regulated by Skp2 in a cell cycle–dependent manner. As predicted, p27Luc levels increased after blocking Cdk2 activity with inhibitory proteins, peptides or small interfering RNA (siRNA). Accumulation of p27Luc in response to Cdk2 inhibitory drugs (flavopiridol and R-roscovitine) was demonstrable in human tumor cells in vivo using noninvasive bioluminescent imaging. In theory, the approach …