Both cell lines were found to be HER2-positive, however, in comparison to HER2-positive breast cancer cell lines, typically harboring receptor densities in the range of 50,000 to 1,000,000 receptors/cell [23], receptor densities were determined to be in the range of ~ 5000 mAb bound per cell (see Table 1), and therefore judged as low-expressing cell lines. The gene discussed is ERBB2; the disease is breast carcinoma.