When considering the HER2-positive subpopulation of GEC, the detection of HER2 amplification in ctDNA has been proposed as an optimal tool to overcome the challenge of tumor temporal and spatial heterogeneity, thus being able to predict and monitor responses to anti-HER2 therapies, and to inform about possible mechanisms of resistance [45,52,69,70,71,72]. The gene discussed is ERBB2; the disease is neoplasm.