Transcriptional repressor involved in the regulation of the circadian rhythm by negatively regulating the activity of the clock genes and clock-controlled genes. Acts as the negative limb of a novel autoregulatory feedback loop (DEC loop) which differs from the one formed by the PER and CRY transcriptional repressors (PER/CRY loop). Both these loops are interlocked as it represses the expression of PER1 and in turn is repressed by PER1/2 and CRY1/2. Represses the activity of the circadian transcriptional activator: CLOCK-ARNTL/BMAL1 heterodimer by competing for the binding to E-box elements (5'-CACGTG-3') found within the promoters of its target genes. Negatively regulates its own expression and the expression of DBP and BHLHE41/DEC2. Acts as a corepressor of RXR and the RXR-LXR heterodimers and represses the ligand-induced RXRA/B/G, NR1H3/LXRA, NR1H4 and VDR transactivation activity. Inhibits HNF1A-mediated transactivation of CYP1A2, CYP2E1 AND CYP3A11 (PubMed:30555544). Homodimer (PubMed:15560782). Heterodimer with BHLHE40/DEC1 (PubMed:15560782). Interacts with CIART (PubMed:24736997). Interacts with ARNTL/BMAL1 (PubMed:12397359). Interacts with RXRA (By similarity). Interacts with NR0B2 and HNF1A (PubMed:30555544). Expressed in skeletal muscle, brain and lung. Expressed in a circdadian manner in the liver with a peak at ZT10. Activity profiles and sleep recordings of transgenic mice carrying the mutation Arg-325 show increased vigilance time and less sleep time than control mice in a zeitgeber time- and sleep deprivation-dependent manner.