MicroRNAs (miRNAs) are a new class of short noncoding regulatory RNAs (18-25 
nucleotides) that are involved in diverse developmental and pathologic 
processes. Altered miRNA expression has been associated with several types of 
human cancer. However, most studies did not establish whether miRNA expression 
changes occurred within cells undergoing malignant transformation. To obtain 
insight into miRNA deregulation in breast cancer, we implemented an in situ 
hybridization (ISH) method to reveal the spatial distribution of miRNA 
expression in archived formalin-fixed, paraffin-embedded specimens representing 
normal and tumor tissue from >100 patient cases. Here, we report that expression 
of miR-145 and miR-205 was restricted to the myoepithelial/basal cell 
compartment of normal mammary ducts and lobules, whereas their accumulation was 
reduced or completely eliminated in matching tumor specimens. Conversely, 
expression of other miRNAs was detected at varying levels predominantly within 
luminal epithelial cells in normal tissue; expression of miR-21 was frequently 
increased, whereas that of let-7a was decreased in malignant cells. We also 
analyzed the association of miRNA expression with that of epithelial markers; 
prognostic indicators such as estrogen receptor, progesterone receptor, and 
HER2; as well as clinical outcome data. This ISH approach provides a more direct 
and informative assessment of how altered miRNA expression contributes to breast 
carcinogenesis compared with miRNA expression profiling in gross tissue 
biopsies. Most significantly, early manifestation of altered miR-145 expression 
in atypical hyperplasia and carcinoma in situ lesions suggests that this miRNA 
may have a potential clinical application as a novel biomarker for early 
detection.