Changes to HER2 testing guidelines for breast cancer in 2013 significantly increased the number of patients who test HER2-positive, according to a new study. Cancers that have an excess of HER2 protein or extra copies of the HER2 gene are called HER2-positive and can be treated with drugs like Herceptin that target HER2. HER2 stands for human epidermal growth factor receptor 2.