ERBB2 and neoplasm: For instance, HER2 staining on CTCs may be helpful to identify a subgroup of tumors with HER2 score 0 at immunohistochemistry who may derive benefit from trastuzumab deruxtecan, an anti-HER2 antibody-drug conjugate with high brain penetration which is currently approved only for HER2-positive and HER2-low breast cancer50,51.