NOTCH1 and head and neck squamous cell carcinoma: The effect of siRNA-mediated NOTCH1 knockdown, inhibition of NOTCH1 signaling via DAPT and stimulation with DLL4 on migration and proliferation was examined in three HPV– and three HPV+ HNSCC cell lines (UD-SCC-5 (UD5), UD-SCC-7 (UD7), UPCI-SCC-111 (UP111), UD-SCC-2 (UD2), UPCI-SCC-154 (UP154), and 93-VU-147T (93VU)).