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 (By similarity). Inhibits HNF1A-mediated transactivation of CYP1A2, CYP2E1 AND CYP3A11 (By similarity). Homodimer (By similarity). Heterodimer with BHLHE40/DEC1 (By similarity). Interacts with CIART (By similarity). Interacts with ARNTL/BMAL1 and RXRA (By similarity). Interacts with NR0B2 and HNF1A (By similarity). Highly expressed in the caudate putamen, pineal gland, granular cell layer of the cerebellum, olfactory bulb, piriform cortex, hippocampus and hypothalamic nuclei. Moderately expressed in skeletal muscle, heart. Weakly expressed in lung. Expressed in a circadian manner in the suprachiasmatic nucleus (SCN) of the brain.