Chromatin-binding factor that repress Notch signaling in the absence of Notch intracellular domain by acting as a CBF1 corepressor. Binds to the HEY promoter and might assist, along with NCOR2, RBPJ-mediated repression (By similarity). May be involved in RNA metabolism (By similarity). In concert with CIC and ATXN1L, involved in brain development (PubMed:28288114). Homooligomer (By similarity). Interacts with PQBP1, UBQLN4 and USP7 (By similarity). Interacts with ANP32A (PubMed:9353121). Interacts with CIC (PubMed:17190598). Directly interacts with RBPJ; this interaction is disrupted in the presence of Notch intracellular domain. Interacts with ATXN1L; competes with ATXN1L for RBPJ-binding (PubMed:17322884). Found in a complex with CIC and ATXN1L (PubMed:28288114). Colocalizes with USP7 in the nucleus. Expressed in the cortex and hypothalamus (at protein level). Widely expressed. In brain, the pattern of distribution is limited to neuron populations. Transient expression burst in Purkinje cells as the cerebellar cortex becomes functional (postnatal day 14), and in mesenchymal cells of the developing intervertebral disks of the spinal column. Atxn1 protein levels are directly regulated by Pum1 protein: Pum1 acts by binding to the 3'-UTR of Atxn1 mRNA, affecting Atxn1 mRNA stability and leading to reduced Atxn1 protein levels. The AXH domain is required for interaction with CIC. Ubiquitinated by UBE3A, leading to its degradation by the proteasome. The presence of poly-Gln repeats in trangenic models developed to replicate phenotypes of the spinocerebellar ataxia 1 disease (SCA1) impair ubiquitination and degradation, leading to accumulation of Atxn1 in neurons and subsequent toxicity. Sumoylation is dependent on nuclear localization and phosphorylation at Ser-751. The murine poly-Gln region is very limited in comparison to human ATXN1 and is not polymorphic. Mice with conditional knockouts of either ATXN1-ATXN1L or CIC in the developing forebrain exhibit intellectual disability, hyperactivity, social-behavioral deficits and reduced thickness of upper cortical layers. Different transgenic mouse, containing a poly-Gln region insertion in position 199 have been developed to replicate phenotypes of the spinocerebellar ataxia 1 disease (SCA1) in human (PubMed:10624951, PubMed:12086639). Heterozygous mice with a poly-Gln of 92 residues [92Q] develop the ataxia typical of human SCA1. However, they show only the phenotype associated with dysfunctional Purkinje cells and usually live a normal lifespan (PubMed:10624951). Heterozygous mice with a poly-Gln of 154 residues [154Q] develop a progressive neurological disorder that resembles human SCA1, with motor incoordination, cognitive deficits, wasting, and premature death, accompanied by Purkinje cell loss and age-related hippocampal synaptic dysfunction (PubMed:12086639). Phenotypes are caused by an accumulation of Atxn1 in neurons, exerting toxicity. The expanded poly-Gln tract causes stabilization of Atxn1 and impairs its ubiquitination and subsequent degradation, increasing its abundance in neurons (PubMed:10624951). Belongs to the ATXN1 family.