Abstract: The invention generally relates to the field of individualized medicine. It is a kind of liquid biopsy for individualizing and monitoring treatment in patients with tumors and for diagnosis of tumors. The invention relates to a method for identifying and characterizing subpopulations (subgroups) of circulating tumor cells (CTCs) in a population of CTCs in a biological sample, preferably in a blood sample of the patient. The invention also relates to the quantification of the respective subgroups of CTCs in the population of CTCs in the biological sample. The invention further relates to the use of the method for diagnosis and monitoring of tumors, decisions relating to treatment, surveillance of treatment and prognosis, in particular for all kinds of solid tumors. The invention further relates to the use of the identified CTC subgroups as biomarkers for diagnosis, prognosis and therapeutic treatment.