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Cancer cells. Artwork of cancer cells breaking away from a tumour (lower right). Cells that break off a primary tumour can travel through the bloodstream or the lymphatic system to other sites in the body, forming a new tumour. This process is known as metastasis. The cancer cells are spherical with rough, irregular surfaces and numerous long extensions (filopodia). The filopodia enable the cells to move around and attach to surfaces. Cancer is a disease caused by the uncontrolled replication of cells in the body. The growth of groups of these cells can severely disrupt the normal function of organs, which can be fatal. (KEYSTONE/SCIENCE PHOTO LIBRARY/Str) === ===