Cajal bodies contain cyclin E/cdk2 and the substrate p220(NPAT) to regulate the transcription of histones, which is essential for cell proliferation, however, recent mouse knockout studies indicate that cyclin E and cdk2 are dispensable for these events. Because the CBP/p300 histone acetyltransferase are also known to be involved in cell proliferation, we examined the molecular and functional interactions of p220(NPAT) with the CBP/p300 at the G1/S boundary as cell cycle regulators. The subnuclear localization of p220(NPAT) and CBP/p300 proteins showed that their foci partially overlapped in a cell cycle dependent manner. Overexpression of p220(NPAT) and CBP/p300 cooperatively enhanced G1/S transition and DNA synthesis even without cdk2 phosphorylation site. Finally, molecular alignment analysis indicated that p220(NPAT) contains several potential substrate sites for CBP/p300. Overall, our findings demonstrate that p220(NPAT) and CBP/p300 form a transient complex at the G1/S boundary to play cooperative roles to promote the S-phase entry.