All of the HER2-targeted drugs mentioned above are now approved for the treatment of breast cancer, but their efficacy against other HER2-positive cancers varies; although there has been a study in which HER2-targeted therapy showed promising efficacy similar to that of trastuzumab for salivary gland cancers [10], there is also the case that HER2-targeted therapy effective to HER2-positive breast cancer is not effective to other HER2-positive cancers, such as T-DM1 to gastroesophageal cancer [30]. Here, ERBB2 is linked to breast cancer.