We evaluated in vitro the cytotoxicity of the best FDCs H0C2.Sx-MMAF (x = 3 to 6) and their corresponding native scFv, compared to our previous FDC 4D5.2-MMAF and scFv 4D5.2, on two human breast cancer cell lines: SK-BR-3 (HER2 high expression) and MDA-MB-231 (HER2 low expression). This evidence concerns the gene ERBB2 and breast cancer.