ERBB2 and neoplasm: The aim of this study was to provide additional insights into the treatment efficacy and the dynamics of the tumor mutational landscape through sequential tissue biopsies and circulating tumor DNA (ctDNA) obtained at diagnosis, before, during, and after the HER2-directed treatment in patients with ERBB2-mutated solid tumors included in the Copenhagen Prospective Personalized Oncology (CoPPO– NCT02290522) trial at our institution [16].