Cullins assemble a potentially large number of ubiquitin ligases by binding to the RING protein ROC1 to catalyse polyubiquitination, as well as binding to various specificity factors to recruit substrates. Cullin 4a is an essential component of the SCF (SKP1-CUL1-F-box protein) E3 ubiquitin ligase complex, which mediates the ubiquitination of proteins involved in cell cycle progression, signal transduction and transcription. In the SCF complex, cul4A serves as a rigid scaffold that organizes the SKP1-F-box protein and RBX1 subunits. Cul4A may also contribute to catalysis through positioning of the substrate and the ubiquitinconjugating enzyme. Cul4A also interacts with RNF7 and is part of a complex with TIP120A/CAND1, Cyclin E and RBX1.