CDKN1B and head and neck squamous cell carcinoma: Thus, given the crucial roles of both p27 and p21 in cell cycle regulation, we hypothesize that p27 (T109G) polymorphism individually or in combination with p21 (C98A and C70T) polymorphisms modifies risk for SPM after index SCCHN, and we tested this hypothesis in a cohort of 1,292 SCCHN patients.