The SCF ubiquitin E3 ligase regulates ubiquitin-dependent proteolysis of many regulatory proteins such as p27(Kip1), IkappaB, and beta-catenin. We report the isolation of a CUL1 binding protein, p120(CAND1). We found the majority of CUL1 is in a complex with CAND1 and ROC1 independent of SKP1 and F box protein SKP2. Both in vivo and in vitro, CAND1 prevents the binding of SKP1 and SKP2 to CUL1 while dissociation of CAND1 from CUL1 promotes the reverse reaction. Neddylation of CUL1 or the presence of SKP1 and ATP causes CAND1 dissociation. Our data suggest that CAND1 regulates the formation of the SCF complex, and its dissociation from CUL1 is coupled with the incorporation of F box proteins into the SCF complex, causing their destabilization.