Although the usefulness of copy number (CN) alterations (e.g., KIT, cyclin-dependent kinase 4, and cyclin D1) in cell-free DNA, including mucosal melanoma (n = 7), has been reported, the applicability of this approach was confined to cases in which a high CN ratio was observed in the primary tumor.15) Therefore, our findings indicated that if tumor-specific somatic mutations can be identified, our method may be applicable to any type of rare tumor. The gene discussed is CDK4; the disease is neoplasm.