Component of a cullin-RING-based BCR (BTB-CUL3-RBX1) E3 ubiquitin-protein ligase complex that mediates the ubiquitination of target proteins, leading most often to their proteasomal degradation. Protein modification; protein ubiquitination. Homodimer. Part of cullin-RING-based BCR (BTB-CUL3-RBX1) E3 ubiquitin-protein ligase complexes that contain CUL3 and SPOP, plus a target protein (By similarity). The BTB (POZ) domain mediates dimerization and interaction with CUL3. The MATH domain mediates interaction with protein-ubiquitin ligase substrates. Belongs to the Tdpoz family.