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. . This is the function of KCTD9 (potassium channel tetramerization domain containing 9, Ensembl gene identifier ENSG00000104756).