BC200 RNA, a small functional RNA that operates as a translational modulator, has been implicated in the regulation of local synaptodendritic protein synthesis in neurons. Cell type-specific expression of BC200 RNA is tightly controlled such that the RNA is not normally detected in somatic cells other than neurons. However, the neuron-specific control of BC200 expression is deregulated in a number of tumors. We here report that BC200 RNA is expressed at high levels in invasive carcinomas of the breast. In normal breast tissue or in benign tumors such as fibroadenomas, in contrast, we found that the RNA is not detectable at significant levels. The difference in expression levels between invasive carcinomas and normal/benign tissue was statistically highly significant. Receiver Operating Characteristics analysis of sensitivity and specificity confirmed the diagnostic power of BC200 RNA as a molecular marker of invasive breast cancer. In ductal carcinomas in situ, furthermore, significant BC200 expression was associated with high nuclear grade, suggesting that the presence of BC200 RNA in such tumors may be used as a prognostic indicator of tumor progression. The combined results demonstrate the potential of BC200 expression to serve as a molecular tool in the diagnosis and/or prognosis of breast cancer.