Abstract only 223 Background: Recently, we identified a six gene panel (CCNE2, DKFZp762E1312, EMP2, MAL2, PPIC, and SLC6A8) for the RT-qPCR based detection of circulating tumor cells (CTC) in breast cancer patients. The aim of the present study was to evaluate the gene panel in further blood samples. Methods: Blood samples were taken from breast cancer patients with metastatic disease (MBC, N=10) or with no evidence of disease (NED, N=30). Putative CTC were enriched by Oncoquick density gradient centrifugation. Total RNA was isolated with RNeasy Micro Kit (QIAgen). Template cDNA was generated with M-MLV Reverse Transcriptase, RNase H Minus (Promega) and random nonamers as primers. RT-qPCR was performed in duplicate reactions using TaqMan Assays (Applied Biosystems) with default thermal cycling parameters. Raw data were analyzed with the AB7900 Sequence Detection Software version 2.2.2 using automatic baseline correction and manual cycle threshold setting. Gene expression was normalized to GAPDH expression. A threshold value T X for each gene X was set at two standard deviations above the mean dCt X value in the healthy control group. A patient was defined as CTC-positive, if at least one gene marker was over-expressed compared to the defined threshold. Results: The gene panel consisting of CCNE2, DKFZp762E1312, EMP2, MAL2, PPIC, and SLC6A8 identified 4/11 MBC but only 5/27 NED patients as CTC positive (p=0.163). By adding known CTC markers (SCGB2A2, TFF1, FXYD3, AGR2, S100A18, and EPCAM) to the panel, 7/11 MBC but only 6/27 NED patients were CTC positive (p=0.018). The presence of CTC in NED patients correlated with pN staging (p=0.026). Only one out of the six CTC positive NED patients relapsed within the observation period (median 35 months, range 25-39 months from blood sampling). We observed no correlation of CTC positivity and recurrence in NED patients. Conclusions: The sensitivity of the RT-qPCR based CTC detection in breast cancer patients may be enhanced by adding known CTC markers (SCGB2A2, TFF1, FXYD3, AGR2, S100A18, and EPCAM) to the six gene panel (CCNE2, DKFZp762E1312, EMP2, MAL2, PPIC, and SLC6A8). Longer follow-up times are needed to evaluate the predictive value of the gene markers on survival.