NOTCH1 and head and neck squamous cell carcinoma: Initially, the activated form of wt NOTCH1 (i.e., ICN1) was stably re-expressed in the NOTCH1 mutant HNSCC cell lines HN4, PCI-15B, UMSCC-47, and HN31 using a green fluorescent protein-tagged bicistronic retroviral vector (MigR1-ICN1).