CDK4 and pancreatic neuroendocrine tumor: In addition, the present study also identified the CNVs of different genes in the CSF ctDNA samples, and the most affected ones were CDKN2A, CDK4 and MDM2. As it is known, the deletion of tumor suppressor CDKN2A was associated with melanoma and pancreatic neuroendocrine tumors metastasis, and the reduced survival rate of patients [30, 31].