Liquid biopsy enables the identification of tumour mutations responsible for tumour response to treatment (i.e. epidermal growth factor receptor [EGFR] in NSCLC), primary or acquired resistance to treatment (i.e. RAS gene mutation in colorectal cancer), and monitoring of tumour dynamics during treatment (i.e. KRAS gene mutation in pancreatic cancer). The gene discussed is EGFR; the disease is neoplasm.