Significant targeted molecules for breast cancer treatment are hormone receptor and HER2. Since the concept of an "intrinsic subtype" was proposed, breast cancers have been treated according to subtypes including Luminal A, B, HER2 and triple negative. In this article, gene testing useful as a prognostic factor and to predict the chemotherapeutic effect on ER-positive breast cancers will be introduced, although it has not been approved in Japan. Gene testing is useful but expensive so far; therefore, it will be used in parallel to conventional tools, i.e. HE-stained sections and immunohistochemical methods.