KAT2B (UniProtKB/Swiss-Prot entry Q92831) functions as a histone acetyltransferase, thereby promoting transcriptional activation. KAT2B binds with CBP and p300 where it competes with the adenoviral oncoprotein E1A for binding sites and as such counteracts the mitogenic activity of E1A. KAT2B also acts as a circadian transcriptional coactivator which enhances the activity of the circadian transcriptional activators NPAS2-ARNTL/BMAL1 and CLOCK-ARNTL/BMAL1 heterodimers.