ERBB2 and neoplasm: •69% (22/32) objective response rate•Significant ERBB2 (HER2) amp discordance, 40% (4/10), between primary tumor and metastasis based on tissue testing•Detectable ERBB2 copy number amplification in plasma at baseline was predictive to the response (100% response rate) and changes in plasma-detected genomic alterations were associated with lapatinib sensitivity and/or resistance