Liquid biopsy with cfDNA is a recommended alternative method of re-biopsy, and detection of epithelial growth factor receptor (EGFR) mutations with cfDNA in patients with lung cancer has been approved as a companion diagnostic for EGFR tyrosine kinase inhibitors (EGFR-TKI) in Japan as well as in the US and Europe [2]. Here, EGFR is linked to lung cancer.