Sirtuins, and SIRT1 in particular, establish a variety of functional interactions with other proteins implicated in cellular metabolism and signaling (Bordone and Guarente 2005). At the level of chromatin, SIRT1 enzymatic activity targets preferentially histone H3 at Lys9 and Lys14, and histone H4 at Lys16 (Imai et al. 2000). In addition, a number of nonhistone proteins are regulated by SIRT1-mediated deacetylation, including p53, FOXO3, PGC-1α, and LXR (Sauve et al. 2006), stressing the pivotal function that this regulator plays in cellular control and responses. We have found that SIRT1 readily deacetylates BMAL1, contributing to its cyclic acetylation levels (Hirayama et al. 2007; Nakahata et al. 2008, 2009). We have generated an antibody that recognizes BMAL1 only when acetylated at the unique Lys537 residue. This anti-Ac-BMAL1 antibody demonstrates that acetylation is induced by CLOCK and that it is highly specific, as demonstrated using a K537R mutant (Fig. 3A,B). Using this antibody we could show that BMAL1 deacetylation occurs in the presence of both SIRT1 and NAD+ and is inhibited by nicotinamide (Fig. 3C). In a series of additional experiments, it was demonstrated that acetylation is circadian and it controls the efficient repression by CRY proteins (Hirayama et al. 2007). These findings have been recently corroborated by quantitative analyses of the BMAL1-CRY interaction by fluorescence polarization and isothermal titration calorimetry (Czarna et al. 2011).