Transcriptional repressor involved in the regulation of the circadian rhythm by negatively regulating the activity of the clock genes and clock-controlled genes (PubMed:18411297). 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) (PubMed:14672706). Both these loops are interlocked as it represses the expression of PER1/2 and in turn is repressed by PER1/2 and CRY1/2 (By similarity). Represses the activity of the circadian transcriptional activator: CLOCK-ARNTL/BMAL1|ARNTL2/BMAL2 heterodimer by competing for the binding to E-box elements (5'-CACGTG-3') found within the promoters of its target genes (By similarity). Negatively regulates its own expression and the expression of DBP and BHLHE41/DEC2 (PubMed:14672706). Acts as a corepressor of RXR and the RXR-LXR heterodimers and represses the ligand-induced RXRA and NR1H3/LXRA transactivation activity (PubMed:19786558). May function as a transcriptional factor for neuronal differentiation (PubMed:9284045). Represses the transcription of NR0B2 and attentuates the transactivation of NR0B2 by the CLOCK-ARNTL/BMAL1 complex (By similarity). Drives the circadian rhythm of blood pressure through transcriptional repression of ATP1B1 in the cardiovascular system (PubMed:30012868). Homodimer. Heterodimer with BHLHE41/DEC2. Interacts with ubiquitin-conjugating enzyme UBE2I/UBC9. Interacts with HDAC1, SUMO1, RXRA and ARNTL/BMAL1 (By similarity). Interacts with TCF3/E47. Predominantly localized in the nucleus (By similarity). Expressed from 9.5 dpc to 17.5 dpc in the ventricular layer of the brain and spinal cord, but also in the retinal pigment epithelium, developing eyelids, nasal epithelium, serous gland, vibrissae, epithelium of the mouth cavity and the tooth buds. Highly expressed in the heart, thymus and adrenal glands followed by lung, liver parenchyma, kidney tubules, epithelium of the esophagus and stomach. From 15.5 dpc to 17.5 dpc it is expressed in urinary bladder and urethra. From 17.5 dpc, it is expressed in developing muscle. Stimulated by retinoic acid (RA). Expressed in a circadian manner in the liver with a peak at ZT10. Ubiquitinated; which may lead to proteasomal degradation. Sumoylation inhibits its ubiquitination and promotes its negative regulation of the CLOCK-ARNTL/BMAL1 heterodimer transcriptional activator activity.