Background: Breast cancer is the most common cancer in women. Non-coding RNAs especially miRNAs have important regulatory roles in cancer. MiRNAs are 21-24 nucleotides which have different levels of expression between tumors and normal tissues. In this study, we have analyzed expression level of miR-520d in three different groups of breast cancer.
Methods: Fifty nine samples were divided into different groups according to their immunohistochemistry (IHC) classification: estrogen receptor (ER) positive and/or progesterone receptor (PR) positive group (as group I); human epidermal growth factor receptor 2 (HER2) positive group (as group II); and Triple negative group (as group III). After small RNA extraction from tissues, cDNAs were synthesized and Real time RT-PCR carried out using DNA binding dye. Expression levels were analyzed by LinRegPCR and REST software.
Results: MiR-520d under- expressed in all of three different groups. The expression ratio in groups I ,II, and III were 0.193, 0.167, 0.21, respectively, but only the result from group II was significant (P=0.017). According to the different clinicopathological status of breast cancer, miR-520d underexpressed significantly not only in patients with metastatic lymph node (P=0.019) but also in patients which have cancer at stage III (P=0.036). 
Conclusion: In this study, we found that miR-520d possibly acts as a tumor suppressor. It may be useful for diagnosis of tumor from normal tissue. In addition, miR-520d significantly underexpressed in HER-2 positive group of breast cancers. Therefore, it may be useful as an additional diagnostic test in this group of breast tumors along with other biomarkers.