In a clinical trial that enrolled confirmed HER2-positive patients, HER2 ITH was assessed by central pathology review and defined as either: (1) HER2 positivity by ISH in > 5% and < 50% of tumor cells (i.e., CAP guideline) or (2) an area of the tumor that tested HER2 negative in at least one of the six areas evaluated per tumor30. The gene discussed is ERBB2; the disease is neoplasm.