The immunocytochemical detection of isolated disseminated tumor cells (ITC) in the bone marrow of breast cancer patients, what is called minimal residual disease (MRD), has been demonstrated to be of prognostic value in all stages of the disease. In order to definitely prove the origin of these cells from the primary tumor it is necessary to identify common factors on both tumor tissue and ITC, furthermore a more detailed characterization could help to improve their prognostic impact by defining certain subgroups and possibly establish new therapeutic strategies. We examined the expression/amplification of HER2neu, CD 44 adhesion molecule and CD 31 angiogenetic factor on more than 200 primary tumor tissues by immunohistochemistry or fluorescence in situ hybridisation, resp., and found no sign. correlation with the detection of ITC. After a median follow-up of 32 months only ITC in the bone marrow were of prognostic significance. In a small number of patients we examined the expression of topoisomerase II alpha, a key enzyme of DNA replication, and its predictive value of eliminating ITC by anthracyclin based chemotherapy. No correlation with the presence of ITC before or after chemotherapy could be found, yet pat. with topoisomerase II alpha neg. tumors showed a trend to reduced disease free survival. Because of the very low number of ITC per bone marrow sample, the direct characterization of these factors on ITC stays difficult without the possibility of tumor cell enrichment or cell culture. Preliminary results on multi colour stained samples indicate that a selection of certain biological factors takes place during tumor cell dissemination.