Approximately one year after successful treatment with cytotoxic chemotherapy and radiotherapy, patients with advanced Small Cell Lung Cancer (SCLC), which primarily affects heavy smokers, generally relapse with recurrence of tumors that are resistant to further chemotherapy. At this point, the affected patients usually only have a few months to live. The reason for this was hitherto unknown. Researchers have now discovered that circulating tumor cells combine, making previously chemo-sensitive cells into chemo-resistant cell complexes.