The phase 2 CHRONOS trial in metastatic colorectal cancer (mCRC) provided proof-of-principle that circulating tumour DNA (ctDNA) could be used to guide therapeutic re-challenge with the anti-epidermal growth factor receptor (EGFR) monoclonal antibody, panitumumab, when resistance-associated mutations in RAS, BRAF and EGFR appeared in ctDNA44. Here, EGFR is linked to metastatic colorectal cancer.