ERBB2 and neoplasm: The HER2 mRNA quantification method (RT-PCR) is robust to variable tissue composition and should yield reliable results in various types of biopsies, and HER2 expression tends to remain stable during the course of the disease.12,13 However, discordant HER2 expression results by IHC between primary tumor and metastatic biopsies can be seen, and Oncotype DX and its components have not been validated for metastatic biopsies.