Humanized bispecific anti-HER2 antibodies that comprise one antigen binding site containing variable regions of heavy and light chain of trastuzumab, and another antigen binding site containing variable regions of heavy and light chain of pertuzumab. The bispecific anti-HER2 antibody is effective for treating cancer, such as breast cancer, gastric cancer, or ovarian cancer. Preferred bispecific anti-HER2 antibodies are afucosylated antibodies. Also provided Chinese Hamster ovary(CHO) mutant cell line that has a dysfunctional Slc35C1 gene, which is the only dysfunctional gene in the mutant that affects glycan regulation.
