**2. Breast cancer classification**

Nowadays, beside conventional use of grade, histology, and immunohistochemical analysis, changes in gene expression during bearing tumors are used as an instrument to classify breast cancer. Molecular profiling make us capable for better understanding of breast cancer, more precision in determining subtypes and better prediction of clinical outcome and response to therapy. New instruments like microarray kits provide the possibility for simultaneous studying of the expression of thousands of genes in a breast cancer cells and finding out the Gene expression profile. Future applications will take the same approach to proteins (proteomics), genome-wide germline variability (single nucleotide polymorphisms), or cellular metabolism (metabolomics). Based on these methods, several distinct breast cancer subtypes have been identified including two main subtypes of estrogen receptor (ER)-negative tumors and basal-like and human epidermal growth factor receptor-2 (HER2)-enriched, and two subtypes of ER-positive tumors including luminal A and luminal B. These subtypes differ markedly in prognosis and in the therapeutic targets they express.

The luminal cancers, luminal A and luminal B, so called because they are characterized by expression of genes also expressed by normal breast luminal epithelial cells, have overlap with ER-positive breast cancers. There are also several subtypes characterized by low expression of hormone receptor-related genes (ER-negative), one of which is called the "HER2-enriched" subtype (previously called HER2+/ER-) and another called the "basal-like" subtype. The basal-like subtype is named because it expresses many genes characteristic of normal breast basal epithelial cells.
