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. KCTD5 (potassium channel tetramerisation domain containing 5) is a 234 amino acid protein that localizes predominantly in the cytoplasm but translocates to the nucleus upon interaction with REP proteins. Existing as a homopentamer and consisting of one BTB (POZ) domain, KCTD5 associates with GRASP55, CUL-3 and ubiquitinated proteins. Interaction with CUL-3 suggests KCTD5 functions as a substrate adapter protein in some E3 ligase complexes.