ERBB2 and neoplasm: Our tumour microenvironment biomarkers determining tumour-directed drug delivery complement tissue biomarkers already available in the clinic for patient stratification in case of actively targeted therapeutics (for example, HER2 staining in case of intended treatment with the HER2-targeted antibody–drug conjugates adotrastuzumab emtansine (Kadcyla) or trastuzumab deruxtecan (Enhertu)).