KCTD9 functions as a substrate-specific adapter protein in BCR (BTB-CUL3-RBX1) E3 ubiquitin-protein ligase complexes, mediating protein ubiquitination and proteasomal degradation 1. The protein forms pentameric assemblies through its BTB domain and binds to Cullin3 (Cul3) to form functional E3 ligase complexes 1. In colorectal cancer, KCTD9 acts as a tumor suppressor by promoting β-catenin degradation through polyubiquitination, thereby inhibiting Wnt signaling and reducing cancer cell proliferation and metastasis 2. KCTD9 associates with ZNT9 to disrupt β-catenin-ZNT9 interactions, leading to decreased Wnt target gene expression 2. The protein's function is regulated by post-translational modifications, as TAF1-mediated β-hydroxybutyrylation of KCTD9 at K123 and K129 promotes TRIM21 binding and subsequent KCTD9 degradation 3. In immune cells, KCTD9 is essential for natural killer (NK) cell development, maturation, and effector function, regulating transcription factors including Ets1, Nfil3, Eomes, and Id2 4. KCTD9 expression correlates with better prognosis in colorectal cancer and enhances immunotherapy response by promoting CD8+ T-cell infiltration 5. Clinically, KCTD9 downregulation is associated with poor outcomes in colorectal cancer patients, making it a potential prognostic biomarker and therapeutic target 26.