Cullin 4B/CUL4B encodes a scaffold protein that organizes a cullin-RING (really interesting new gene) ubiquitin ligase (E3) complex in ubiquitylation. The CUL4 gene encodes a protein of 913 amino acids. The cullin domain is located between amino acid residues 217 and 815 and is characterized a by C-terminal globular domain (cullin homology domain) and a series of N-terminal repeats (cullin repeats). Ohtake et al. (2007) characterize a fat-soluble ligand-dependent ubiquitin ligase complex in human cell lines, in which dioxin receptor (AhR) is integrated as a component of a novel cullin 4B ubiquitin ligase complex, CUL4B(AhR). Complex assembly and ubiquitin ligase activity of CUL4B(AhR) in vitro and in vivo are dependent on the AhR ligand. In the CUL4B(AhR) complex, ligand-activated AhR acts as a substrate-specific adaptor component that targets sex steroid receptors for degradation. Their findings uncover a function for AhR as an atypical component of the ubiquitin ligase complex and demonstrate a non-genomic signalling pathway in which fat-soluble ligands regulate target-protein-selective degradation through a ubiquitin ligase complex.