Thus, results presented in this study are consistent with a model in which the initial activation of caspase-9 is amplified via caspase-8-mediated positive feedback loop, which is required for the full activation of the caspases in p53/p73-dependent apoptosis induced by etoposide in HNSCC cells (Figure 8A). Here, TP53 is linked to head and neck squamous cell carcinoma.