Invasive breast cancers can be classified into three major subtypes on the basis of gene expression: luminal, basallike and Her2/neu-overexpressing.Many investigators have used surrogate immunohistochemical markers to classify these tumors; ER, PR, and HER2 negative breast cancers (Triple Negative breast cancer Profile=TNP) are classified as normal breast like if basal cytokeratins and EGFR are lacking, and basal-cell like cancers=BLC when basal cytokeratin (CK5-6 and/or CK14) are expressed.Most breast cancer studies contain 8-15% TNPs and 10-15% BLCs and mortality in these cancers is high (30-50% in lymph node negative/TNP breast cancers).It has been shown that TNP and basal like breast cancers are often (but not always) associated with high proliferation, high grade, young age, BRCA1 and aggressive clinical behavior.Recently an extra level of gene regulation was discovered, small non-coding RNA molecules were found to play an important role in gene-silencing by binding directly and specifically to mRNA molecules and enabling there degradation.These small RNA molecules called microRNAs, are 19-25 nucleotides in length and compose the largest family ofnoncoding RNAs involved in gene silencing.In the current study we performed a microRNA array study on 103 lymph node negative breast cancers in order to investigate the correlation between microRNA expression and several biologic features of breast cancer like proliferation, and ER/PR/Her2/CK5&6 expression as a step in understanding more about the fundamentals of the regulation of these important breast cancer features.