ERBB2 and melanoma: To further investigate the extent to which the proposed mechanism of action by the BsAb leads to enhanced antitumor activity, we implanted D5-HER2 syngeneic melanoma cells subcutaneously in hmHER2Tg mice, a transgenic mouse line in the C57BL/6 background that is immunocompetent but immunotolerant to HER2 (31–33).