CDK4 and head and neck squamous cell carcinoma: CDK4 is a well-known cancer treatment target, and CDK4 inhibitors (e.g., abemaciclib and palbociclib) have demonstrated cytostatic activity in HPV-negative HNSCC, whereas their effects on HPV-positive HNSCC are not obvious (Adkins et al., 2019; Oppelt et al., 2021; Robinson et al., 2019).