MTOR and head and neck squamous cell carcinoma: For example, when faced with an HNSCC patient who failed multi-line therapies and the gene sequencing results showed that the key enzyme of purine metabolism-ADA is highly expressed, clinicians can try to use novel small-molecule inhibitors, such as AT-7519 (CDK inhibitor), AZD7762 (CHK inhibitor), and AZD8055 (mTOR inhibitor) for treatment.