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Regulatory brain cytoplasmic 200 RNA (BC200 RNA) is highly expressed in human mammary carcinoma cells. Here, we ask whether BC200 RNA becomes detectable in peripheral blood of patients with invasive breast cancer. Using quantitative reverse-transcription PCR (qRT-PCR) methodology, we observed that BC200 RNA blood levels were significantly elevated, in comparison with healthy subjects, in patients with invasive breast cancer prior to tumorectomy (p=0.001) and in patients with metastatic breast cancer (p=0.003). In patients with invasive breast cancer who had recently undergone tumorectomy, BC200 RNA blood levels were not distinguishable from levels in healthy subjects. However, normality analysis revealed a heterogeneous distribution of patients in this group, including a subgroup of individuals with high residual BC200 RNA blood levels. In blood from patients with invasive breast cancer, BC200 RNA was specifically detected in the mononuclear leukocyte fraction. The qRT-PCR approach is sensitive enough to detect as few as three BC200 RNA-expressing tumor cells. Our work establishes the potential of BC200 RNA detection in blood to serve as a molecular indicator of invasive breast malignancy.