The aim of this study is to identify a gene expression signature which is characteristic of ER status in breast cancer patients. To our knowledge, this is the first microarray study in Greece involving clinical samples. We identified 97 genes that are characteristic for ER status and can well distinguish the ER+ from the ER- samples. We shrank our list to a 11-gene list correlating to the same patient ER status. We found a significant overlap of these genes with published ER status characteristic signatures like the ones of West et.al.  and of Van't Veer et. al. . This fact is very important given the minimal overlap of such genes reported by others . In order to obtain a molecular insight into how the expression of estrogen receptor activates cancer cells, we found associations with biological pathways. Interestingly, the vast majority of these genes are highly related to breast cancer.