Once researchers knew that cancer cells could release molecules to activate the process of angiogenesis, the challenge became to find and study these angiogenesis-stimulating molecules in animal and human tumors.

From such studies more than a dozen different proteins, as well as several smaller molecules, have been identified as "angiogenic," meaning that they are released by tumors as signals for angiogenesis. Among these molecules, two proteins appear to be the most important for sustaining tumor growth: vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). VEGF and bFGF are produced by many kinds of cancer cells and by certain types of normal cells, too.