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Minimal residual disease (MRD) evaluation in breast cancer patients is a promising tool to improve current staging procedures. In a previous work employing a CK-19—based reverse transcriptase—polymerase chain reaction (RT-PCR) technique for MRD detection, we identified a group of women who exhibited persistent negativity for this assay and for whom this technique was considered noninformative. In order to improve the yield of MRD detection in these patients, we evaluated the usefulness of RT-PCR detection of c-erbB-2 expression. We were able to detect up to 1 MCF-7 cell (positive for c-erbB-2expression) in a mixture of 1,000,000 CCRF-CEM cells (negative for c-erbB-2expression). We evaluated the specificity of this technique in the peripheral-blood mononuclear cells (PBMCs) of 20 healthy women and found that 2 of these women were positive for c-erbB-2expression. In the PBMCs of a group of 16 women with breast cancer, 25% of the samples were positive for c-erbB-2expression before chemotherapy. Except for race (P= 0.017), no other significant correlations were found, including c-erbB-2expression in the primary tumor by immunoperoxidase. Interestingly, in the subgroup of 6 patients for whom this technique was informative, we found that 80% of the samples obtained while on chemotherapy were negative compared to only 10% obtained off treatment (P= 0.017). Additionally, 2 patients for whom CK-19expression was noninformative had at least 1 c-erbB-2—positive sample. We conclude that this technique might be useful for MRD detection in breast cancer patients, but further studies are necessary to confirm our findings.