The cluster aims at the early and reliable detection of circulating tumor cells/MRD in peripheral blood and bone marrow samples of patients bearing selected solid tumors. In addition to confirmation of residual disease, specific markers (gene expression and methylated gene detection) linked to the metastatic potential of the respective tumor cells should be investigated for their prognostic value. Application of the techniques for the highly sensitive detection of remaining tumor cells in samples derived from patients treated with a variety of innovative and experimental anticancer drugs is expected to yield information about the efficacy of different drugs to eliminate residual metastatic tumor cells. Cultivation of primary tumor cells would allow for characterization of the cell biology of the recurrent tumor cells, in respect to gene expression, heterogeneity, growth factor dependency, chemosensitivity anid invasiveness. In summary, the study of disseminated tumor cells/MRD using gene technology, molecular and cell biology methods in combination with clinical cancer therapy in this cooperative cluster effort targets one of the most urgent questions in the treatment of tumors: the early detection, the assessment of the prognosis and the prevention of tumor recurrence deriving from residual circulating tumor cells in patients that apparently responded to primary treatment. Earlier diagnosis and improved treatment of residual disease in cancer patients is expected to result in reduced rates of clinical recurrence and improved survival. This aim is in accordance with the EU long-term goal to change the status of cancer from a metastatic fatal disease to a non-fatal chronic disease.