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. Binds RNA in vitro. May be involved in RNA metabolism. In concert with CIC and ATXN1L, involved brain development. Homooligomer. Interacts with CIC. Interacts with ANP32A, PQBP1, UBQLN4, ATXN1L and USP7. Directly interacts with RBPJ; this interaction is disrupted in the presence of Notch intracellular domain. Competes with ATXN1L for RBPJ-binding. Found in a complex with CIC and ATXN1L. Colocalizes with USP7 in the nucleus. The AXH domain is required for interaction with CIC. Ubiquitinated by UBE3A, leading to its degradation by the proteasome. Phosphorylation at Ser-749 increases the pathogenicity of proteins with an expanded polyglutamine tract. Sumoylation is dependent on nuclear localization and phosphorylation at Ser-749. It is reduced in the presence of an expanded polyglutamine tract. The rat poly-Gln region is very limited in comparison to human ATXN1 and is not polymorphic. Belongs to the ATXN1 family.