CDKN1A and head and neck squamous cell carcinoma: It seems that, in the heterogenic HNSCC tumor cell nests and cultured cell lines, several survival strategies are present at the same time: one of them is the autocrine NGF–NTRK1 system, and a possible other is the NGF-independent (since p75NTR+ cells have low or no NTRK1) system, which offer a therapy resistance over p21/WAF1.