We tested a series of drugs commonly used for HER2‐positive breast cancer treatment including one anti‐HER2 monoclonal antibody, two anti‐HER2 tyrosine kinase inhibitors, one PI3K inhibitor, two AKT inhibitors, and two chemotherapies for their antiproliferative effects in MDA‐MB‐453, HCC1954, and CK‐MB‐1 cells. This evidence concerns the gene PIK3CA and breast cancer.