ATM and head and neck squamous cell carcinoma: In addition, a recent circulating tumor (ctDNA) sequencing study of 75 patients with HNSCC found that the five most commonly altered genes were TP53, CDKN2A, TERT, BRCA2, and NOTCH1, and that 38.8% of patients had alterations in DNA repair genes (APC, ATM, BRCA1, or BRCA2) (29).