Cullin-associated and neddylation-dissociated (CAND1)/TIP120A is a protein containing multiple HEAT repeats. It functions, in part, as an inhibitor of multiple cullin-RING ubiquitin ligases (CRLs) via binding to cullin-RBX complexes that are both unconjugated to NEDD8 and lack association with substrate recognition subunits (1-3). Indeed, CAND1 has been shown to bind all cullin family members in human cells and analysis of the crystal structure of human CAND1 bound to the CUL1-RBX1 complex suggests that CAND1 inhibits the activity of CRLs by sterically blocking both the substrate recognition subunit binding site and the NEDD8 conjugation site . Conversely, CAND1 binding to cullin-RBX complexes is incompatible with neddylation as NEDD8 conjugated to cullins blocks CAND1 binding, suggesting that CAND1 binds to cullins only after the COP9 signalosome has catalyzed cullin deneddylation. Through its ability to negatively regulate CRL assembly, CAND1 plays an integral part in facilitating CRL activation cycles that allow CRLs to utilize distinct substrate recognition subunits and protects these subunits from undergoing ubiquitin-dependent degradation .