Substrate-specific adapter of a BCR (BTB-CUL3-RBX1) E3 ubiquitin-protein ligase complex, which mediates the ubiquitination of target proteins, leading to their degradation by the proteasome. Protein modification; protein ubiquitination. Forms pentamers. Component of a complex mades of five KCTD9 and five CUL3 subunits.