RAD51 and head and neck squamous cell carcinoma: A third study attributes the deficiency in DSB repair in two HPV-positive versus one HPV-negative HNSCC cell line to decreased BRCA2, DNA-Pkcs, and 53BP1 protein expression in HPV-positive HNSCC cells, which ultimately led to NHEJ and HR signaling deficiency as demonstrated through the lack of DNA-Pkcs and BRCA2/RAD51 foci, respectively post-irradiation[26].