NOTCH1 and head and neck squamous cell carcinoma: Furthermore, the addition of γ-secretase inhibitor to NOTCH1-knockdown and NOTCH1-knockout SCCHN cells did not boost the phosphorylation of AKT, indicating the AKT-stimulating effect was only associated with NOTCH1 but not other substrates of γ-secretase (Fig. 2D and Fig. 2E, respectively).