CDK4 and head and neck squamous cell carcinoma: However, promising treatment outcomes of CDK4 inhibitors were only observed in HPV-negative HNSCC rather than HPV-positive HNSCC (Adkins et al., 2019; Oppelt et al., 2021; van Caloen and Machiels, 2019) due to the mutation differences of cell cycle-related genes in cancer cells, and less attention has been paid to the effects of CDK4 inhibitors on infiltrating T cells.