The BTB (Broad-Complex, Tramtrack and Bric a brac) domain, also known as the POZ (Poxvirus and Zinc finger) domain, is an N-terminal homodimerization domain that contains multiple copies of kelch repeats and/or C2H2-type zinc fingers. Proteins that contain BTB domains are thought to be involved in transcriptional regulation via control of chromatin structure and function. ZBTB1 (zinc finger and BTB domain containing 1), also known as KIAA0997, is a 713 amino acid nuclear protein that contains one BTB (POZ) domain and 8 C2H2-type zinc fingers. ZBTB2 is a 514 amino acid nuclear protein that contains one BTB (POZ) domain and 4 C2H2-type zinc fingers. ZBTB25, also known as ZNF46 or KUP, is a 435 amino acid nuclear protein that is expressed mainly in hematopoietic cells and testis and contains one BTB (POZ) domain and 2 C2H2-type zinc fingers.