Transcriptional repressor which forms a negative regulatory component of the circadian clock and acts independently of the circadian transcriptional repressors: CRY1, CRY2 and BHLHE41. In a histone deacetylase-dependent manner represses the transcriptional activator activity of the CLOCK-ARNTL/BMAL1 heterodimer. Abrogates the interaction of ARNTL/BMAL1 with the transcriptional coactivator CREBBP and can repress the histone acetyl-transferase activity of the CLOCK-ARNTL/BMAL1 heterodimer, reducing histone acetylation of its target genes. Rhythmically binds the E-box elements (5'-CACGTG-3') on circadian gene promoters and its occupancy shows circadian oscillation antiphasic to ARNTL/BMAL1. Interacts with the glucocorticoid receptor (NR3C1) and contributes to the repressive function in the glucocorticoid response (By similarity). Interacts with PER2, CRY2, BHLHE41, HDAC1 and NR3C1 (By similarity). Interacts with ARNTL/BMAL1. Co-localizes with the CLOCK-ARNTL/BMAL1 heterodimer in the PML body.