Acts as a sequence-specific transcriptional repressor in association with BCL6. Necessary for activation of naive T-cells to antigenic stimulation. May attenuate the regulatory effect of BCL6 on antigenic activation of naive CD4 T-cells by forming a heterodimer with BCL6. Associates with BCL6 through the BTB domain. Ubiquitously expressed with higher expression found in heart and lung. Induced in activated lymphocytes. Amino acids 178-210 are essential for repression activity.