CDKN2A (9%), FAT1 (23%), SMAD4 (11%), and CASP8 (13%) mutations were only present in HPV-negative HNSCC cell lines, which was consistent with the HNSCC TCGA data, where each of these mutations occurred in only 1 HPV-positive HNSCC patient tumor. This evidence concerns the gene SMAD4 and head and neck squamous cell carcinoma.