To assess if ZHER2:2891DCS-MMAE can selectively target HER2 overexpressing tumor cells, we used three different cell lines: the human adenocarcinoma cell line SK-BR-3 and ZR-75-1 that express high levels of HER2 (HER2+) [15,27,28,38], and the human mammary gland adenocarcinoma MDA-MB-231, which expresses low (basal) levels of HER2 (HER2-) [39]. Here, ERBB2 is linked to adenocarcinoma.