Avastin is thought to work by attaching to a protein called vascular endothelial growth factor (VEGF) to block its action. VEGF plays a role in the formation of both normal and abnormal blood vessels. It is present in normal tissues, but is produced in excess by most solid cancers (tumors). In cancer, VEGF helps blood vessels bring nutrients to tumor cells, allowing the tumor cells to grow. In laboratory studies with human cancer cells grown in animals, Avastin has been shown to prevent or slow the growth of different types of cancer cells by blocking the effects of VEGF.