Since the concept of gene profile-based intrinsic subtypes was proposed, various studies on pathological characteristics have been performed. Particularly, triplenegative (TN) breast cancer, which is negative for all hormone receptors [estrogen receptor (ER) and/or progesterone receptor (PgR) and human epidermal growth factor 2 (HER2)], has been attracting attention because effects of endocrine and targeting therapies cannot be anticipated and thus selecting a treatment method is difficult. TN cancer accounts for about 10%-15% of all invasive breast cancer cases in Japanese, which is significantly lower than the incidence reported in the United States. Cytokeratin (CK) 5/6 or epidermal growth factor receptor (EGFR) is positive in 80%, being classified as basal-like carcinoma, but it should be understood that TN breast cancer and basal-like carcinoma are not necessarily the same. Criteria for positivity judgment of ER, PgR, and HER2 were established to select treatment in cases positive for each marker, and greater importance is attached to strict accuracy control. Inversely, the level of negative findings to judge TN varies among the judgment criteria. In any case, the prognosis of TN breast cancer is poor. Pathologically, TN breast cancer shows certain morphological characteristics, such as high grade and a pushing margin, and abnormalities of BRCA1 and p53 are frequently noted. At present, as no effective therapeutic strategy has been established for TN breast cancer, further clarification of the molecular biological characteristics of such cancers is needed. In addition, the incidence of TN-type ductal carcinoma in situ (DCIS) is low, suggesting that TN does not remain preinvasive DCIS for a prolonged period and that it transforms to invasive cancer in an early stage. Because mammary gland basal cells have characters of progenitor or stem cells that differentiate into both luminal epithelium and myoepithelial cells, these cells may be utilized for the differential diagnosis of the benignity or malignancy of intraductal lesions in routine pathological practice. As proliferation markers, such as Ki-67, and multiple gene arrays for gene signature are also utilized to select adjuvant therapy, analysis may progress further in the future.
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