Two speci®c inhibitors of cyclin-dependent kinase 2 (Cdk2), roscovitine and olomoucine, have been shown recently to induce nuclear accumulation of wt p53 and nucleolar unravelling in interphase human untransformed IMR-90 and breast tumor-derived MCF-7 cells. Here, we show that the early response of MCF-7 cells to roscovitine is fully reversible since a rapid restoration of nucleolar organization followed by an induction of p21 WAF1/CIP1 , a downregulation of nuclear wt p53 and normal cell cycle resumption occurs if the compound is removed after 4 h. Interestingly, similar reversible e ects are also induced by the casein kinase II (CKII) inhibitor, 5,6-dichloro-1-b-D-ribofuranosylbenzimidazole. Upon short-term treatment also, both compounds signi®cantly, but reversibly, reduce the level of 45S precursor ribosomal RNA. Cells exposed to the two types of protein kinase inhibitors for longer times keep exhibiting altered nucleolar and wt p53 features, yet they strikingly di erentiate in that most roscovitine-treated cells fail to ever accumulate high levels of p21 WAF1/CIP1 in contrast with DRB-treated ones. In both cases, however, the cells eventually fall into an irreversible state and die. Moreover, we found that constitutive overexpression of p21 WAF1/CIP1 alters the nucleolar unravelling process in the presence of DRB, but not of roscovitine, suggesting a role for this physiological Cdk inhibitor in the regulation of nucleolar function. Our data also support the notion that both roscovitine-and DRB-sensitive protein kinases, probably including Cdk2 and CKII, via their dual implication in the p53-Rb pathway and in ribosomal biogenesis, would participate in coupling cell growth with cell division. Oncogene (2001) 20, 5951 ± 5963.