IFN-γ or IL-4 arrest cell proliferation at the G1/S checkpoint thanks to elevation of the cyclin-dependent kinase inhibitor p21Cip1 (34, 35). We sought to inhibit proliferation at the G1/S checkpoint by an alternative mechanism. Given that gene silencing is inefficient in macrophages, we overexpressed the cyclin-dependent kinase inhibitor p27kip1 (37) from an adenovirus. Infection with the p27kip1 expressing virus increased p27kip1 protein, as expected (Figure 6A), and dramatically decreased BrdU incorporation (Figure 6B), pRb (Figure 6C) and H3pS-10 (Figure 6D). Hyperphosphorylation of Rb releases the S-phase transcription factor, E2F, which induced multiple genes that include Cyclin E and proliferating cell nuclear antigen (PCNA) (38). As expected, therefore, overexpression of p27kip1 profoundly decreased mRNA levels of cyclin E (84 ± 2%) and PCNA (86 ± 6%, both n = 3, p < 0.01), confirming G1/S blockade. Overexpression of p27kip1 decreased mRNA levels of AURKB, ESCO2, SUV39H1, and WHSC1 (Figure 6E), to a similar extent as IFN-γ or IL-4. By contrast, levels of the other genes downregulated by IFN-γ, namely KAT2A, PRMT7, SETD6, and SMYD3 were not significantly reduced by p27kip1 overexpression (Figure 6E) and were, therefore, clearly independent of inhibition of proliferation by either growth factor depletion or p27kip1 overexpression. Steady-state mRNA levels of the genes upregulated by IFN-γ or IL-4, namely CIITA, KDM6B, and NCOA1, were not decreased or even increased by overexpression of p27kip1 (Figure 6E).