A: Cdk1 is required for maintenance of Cdc5 levels in G2. Cdk1as1  cells were synchronized in S-phase with 0.2 M hydroxyurea and released for 100 minutes into G2 (Cdc5 levels are near maximal) in media containing 5 μg/ml nocodazole. 0.5 μM 1 NM-PP1 (from a 1 mM stock in DMSO) or an equivalent volume of DMSO was added. B: As A, except that at 100 minutes 200 μg/ml cycloheximide (CHX) was also added to both samples. The activity of CHX is confirmed by the fact that levels of Clb2 do not continue to rise as happens in its absence (Figure 3A).