Transcriptional corepressor. Mediates the transcriptional repression activity of some nuclear receptors by promoting chromatin condensation; thus preventing access of the basal transcription. Isoform 1 and isoform 5 have different affinities for different nuclear receptors. Involved in the regulation BCL6-dependent of the germinal center (GC) reactions; mainly through the control of the GC B-cells proliferation and survival. {ECO:0000269|PubMed:18212045; ECO:0000269|PubMed:23911289}.