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). Can suppress the cytotoxicity of ATXN1 in spinocerebellar ataxia type 1 (SCA1) (PubMed:16121196). In concert with CIC and ATXN1, involved in brain development (PubMed:28288114). Homodimer. Interacts (via AXH domain) with NCOR2 (By similarity). Interacts with ATXN1 and CIC. Directly interacts with RBPJ; this interaction is disrupted in the presence of Notch intracellular domain. Competes with ATXN1 for RBPJ-binding (By similarity). Found in a complex with CIC and ATXN1 (PubMed:28288114). Forms nuclear foci. Colocalizes with NCOR2 and HDAC3. Distributed beyond the nucleus into the cell body and dendrites in Purkinje cells and in inferior olive cells. Expressed in the cortex and hypothalamus (at protein level). Expressed in neuronal cells. Highly expressed in Purkinje cells of cerebellum. 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. Its overexpression suppresses the ataxia caused by polyglutamine-expanded Atxn1. Competes with mutant Atxn1 and wild-type Atxn1 for association with CIC. Decreased association of mutant Atxn1 into its CIC-containing complexes decreases the levels of mutant Atxn1-containing CIC complexes, suppressing pathology, while promoting aggregation and thus increasing nuclear inclusions. Belongs to the ATXN1 family.