The presence of extracellular DNA in blood, referred to as cell-free DNA (cfDNA), was first reported in 1948.1 The first reports of tumor-specific mutations in cfDNA were published in 1994, when KRAS and NRAS mutations were observed in patients with pancreatic cancer and acute myelogenous leukemia.2,3 Recent work indicates that for many cancers, there are genetic variants present in cfDNA that broadly overlap with variants found in tumor tissue.4 This tumor-derived fraction of cfDNA is commonly referred to as circulating tumor DNA (ctDNA). The gene discussed is KRAS; the disease is neoplasm.