What leads to cyclic acetylation if CLOCK and SIRT1 levels do not oscillate? The levels of K9/K14 H3 acetylation at the Dbp promoter monitored by chromatin immunoprecipitation (Nakahata et al. 2008) and of BMAL1 as monitored using the anti-AcBMAL1 antibody (see Fig. 3) oscillate in a circadian manner. These events are elicited by a complex interplay of circadian regulators, in which CLOCK and SIRT1 appear to have a central role. Ultimately, circadian gene expression (lower panel) may not depend exclusively on proteins but also on the circadian oscillation of metabolites, such as NAD+ (Nakahata et al. 2009; see also Fig. 5).