Transcriptional repressor which plays a role in development of the central nervous system (CNS) (PubMed:17190598). In concert with ATXN1 and ATXN1L, involved in brain development (PubMed:28288114). Isoform 1: Interacts with ATXN1 (PubMed:17190598). Isoform 2: Interacts with ATXN1 (PubMed:17190598). Interacts with ATXN1L (PubMed:17322884). Found in a complex with ATXN1 and ATXN1L (PubMed:28288114). Expressed in the cortex and hypothalamus (at protein level). Isoform 1: Present in cerebellum (at protein level) (PubMed:17190598). Isoform 2: Present in cerebellum (at protein level) (PubMed:17190598). Expressed postnatally in the hippocampus, neocortex, olfactory bulb and cerebellum. Expressed in the hippocampus from P0 to P15. Expressed in the posterior region of the neocortex at P4, the anterior region at P9, and is then lost by P15. Expressed in the cerebellum from P0, specifically within the external granule layer (EGL) which contains granule cell precursors. By P7 expression is distributed throughout the EGL and is reduced in the inner granule cell layer (IGL) where granule cells finally differentiate. Expression in the IGL continues to diminish up to P15, when granule cell neurogenesis is complete. 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. Intron retention.