IHC scoring is based on the following pattern of membrane staining: a score of 3 indicates positive HER2 expression, marked by intense staining of the cell membrane in over 30% of invasive tumor cells; a score of 2 indicates equivocal HER2 protein expression, where membrane staining is complete but may be uneven or faint in intensity, covering at least 10% of cells; and a score of 0 or 1 indicates negative HER2 protein expression, showing no significant staining of the cell membrane. The gene discussed is ERBB2; the disease is neoplasm.