Recent reports have suggested that early (e.g., 8–21 days after treatment initiation) declines in the amount of ctDNA are associated with a tumor response to therapies in metastatic cancer, including anti-EGFR and check point inhibitor therapy in lung cancer [21,22], Palbociclib and Fulvestrant [23], anti-HER2 therapy [24] and AKT inhibitor with Paclitaxel [25] in metastatic breast cancer. This evidence concerns the gene ERBB2 and metastatic malignant neoplasm.