Acts as a transcriptional repressor. Corepressor for REST. Interacts with MXI1 to repress MYC responsive genes and antagonize MYC oncogenic activities. Also interacts with MXD1-MAX heterodimers to repress transcription by tethering SIN3A to DNA. Acts cooperatively with OGT to repress transcription in parallel with histone deacetylation. Involved in the control of the circadian rhythms. Required for the transcriptional repression of circadian target genes, such as PER1, mediated by the large PER complex through histone deacetylation. Cooperates with FOXK1 to regulate cell cycle progression probably by repressing cell cycle inhibitor genes expression (By similarity). Required for cortical neuron differentiation and callosal axon elongation (By similarity). Interacts with ARID4B, BRMS1L, HCFC1, HDAC1, HDAC2, MXI1, SAP30L, SAP130, SFPQ and TOPORS (PubMed:11259580, PubMed:11897684, PubMed:12670868, PubMed:12724404, PubMed:15451426, PubMed:16820529, PubMed:17803295). Interacts with OGT (via TPRs 1-6); the interaction mediates transcriptional repression in parallel with histone deacetylase (PubMed:12150998). Interacts with BAZ2A, MXD1, MXD3, MXD4, MBD2, DACH1, NCOR1, NR4A2, REST, RLIM, SAP30, SETDB1, SMYD2, and SUDS3 (PubMed:14525983). Interacts with PHF12 in a complex composed of HDAC1, PHF12 and SAP30 (By similarity). Interacts with TET1; the interaction recruits SIN3A to gene promoters (PubMed:21490601). The large PER complex involved in the histone deacetylation is composed of at least HDAC1, PER2, SFPQ and SIN3A (By similarity). Interacts with KLF11 (By similarity). Interacts with PPHLN1 (PubMed:17963697). Found in a complex with YY1, GON4L and HDAC1 (By similarity). Interacts (via PAH2) with FOXK1 (By similarity). Interacts with FOXK2 (By similarity). Found in a complex composed of at least SINHCAF, SIN3A, HDAC1, SAP30, RBBP4, OGT and TET1. Interacts with SINHCAF (By similarity). Interacts with SPHK2 (PubMed:19729656). Recruited to the nucleolus by SAP30L. Expressed in the developing brain, with highest levels of expression detected in the ventricular zone of various cortical regions. SUMO1 sumoylated by TOPORS. Probably desumoylated by SENP2. The disease is caused by variants affecting the gene represented in this entry.