CDKN1B and head and neck squamous cell carcinoma: Previously, it was reported that S-phase kinase-associated protein 2 (Skp2), the ubiquitin ligase subunit that specifically targets the negative cell-cycle regulator p27Kip1 for degradation, is overexpressed in various cancers, including human HNSCC; its expression levels are inversely correlated to those of p27Kip1 in these cells27.