KRAS and sigmoid colon cancer: In this report, we describe our observation of genomic alterations in <i>KRAS</i> and <i>NRAS</i> in a patient with acquired resistance to anti-EGFR antibody drugs by tracking clonal evolution using serial ctDNA anaylsis.<h4>Case presentation</h4>A 54-year-old woman was initially diagnosed with sigmoid colon cancer with multiple liver metastases.