RB1-dependent polyubiquitination of SKP2 by the APC/C:Cdh1 complex is an important mechanism of RB1-mediated cell cycle exit, which contributes to the RB1 tumor suppressive role. ABC/C:Cdh1-mediated polyubiquitination targets SKP2 for proteasome-mediated degradation. RB1 and APC/C:Cdh1-dependent degradation of SKP2 allows accumulation of CDKN1B (p27Kip1) in the cell, as CDKN1B is a target of the SKP2-containing SCF ubiquitin ligase complex. CDKN1B acts as a CDK inhibitor, enabling mitotic exit (Ji et al. 2004, Binne et al. 2007).