Animation of a tumour attracting new blood vessels to grow towards it, a process known as angiogenesis. Initially a tumour is a small ball of cancerous cells, which divide rapidly. A tumour can only grow to a certain size without its own blood supply, however, and without its own blood supply it generally remains benign. To increase its supply of oxygen and nutrients, and to help remove its waste, a tumour can secrete growth factors that prompt the development of new vessels. This angiogenesis is therefore a critical stage in the development of a malignant tumour. Drugs that target and inhibit angiogenesis are important in the treatment of cancer.