RAD51 and head and neck squamous cell carcinoma: Incubation of cells with HU caused DNA damage in both HPV+ and HPV-negative HNSCC cells as indicated by the appearance of ɣH2AX foci (Figure 4B); however homologous recombination, as marked by colocalized Rad51 foci, was activated only in HPV-negative cells (Figure 4B).