BTB/POZ domain-containing protein KCTD5, also known as KCTD5, is a 234aa protein that localizes predominantly in the cytoplasm but translocates to the nucleus upon interaction with REP proteins. The expression of KCTD5 was upregulated post-transcriptionally in peripheral blood lymphocytes stimulated through the T-cell receptor. KCTD5 interacted specifically with cullin3, bound ubiquitinated proteins, and formed oligomers through its BTB domain.