Since HER2 overexpression is a significant predictor of both overall survival and time to relapse in patients with breast cancer, (37) the absence of HER2 test results is a significant barrier to conducting comparative effectiveness research (e.g. chemotherapy with versus without trastuzumab) on trastuzumab using this database, because one cannot identify a HER2 positive control group. This evidence concerns the gene ERBB2 and breast cancer.