Hematogenous distant metastasis is the leading cause of cancer-related death in breast cancer and other solid tumors. By applying sensitive immunocytochemical and molecular assays, disseminated tumor cells (DTC) in bone marrow (BM) can be detected in 20-40% of cancer patients without any clinical or even histopathological signs of metastasis and the presence of these DTC at primary diagnosis predicts the subsequent occurrence of overt metastases in bone and other organs. cDNA-microarray analysis on primary breast carcinomas from patients with and without tumor cells in BM revealed a predominant downregulation of potential metastasis-suppressor genes in BM-positive tumors. Thus, dissemination of tumor cells appears to be an early process associated with a specific molecular signature of the primary tumor.