Folkman's first approach to understanding how tumors "turn on" angiogenesis was to search for an actual angiogenic protein produced by a tumor. To make this search possible, Folkman needed research tools and tests that could demonstrate such a protein's presence. The first step was the ability to grow blood vessel endothelial cells in vitro so that vessel growth could be easily monitored under different experimental conditions. This was accomplished for the first time in the early 1970s by Folkman and his student, Michael A. Gimbrone, Jr., M.D., now Chair of Pathology and Director of Vascular Research at Brigham and Women's Hospital. A second tool consisted of tiny polymer beads that could be implanted into the corneas of laboratory animals, releasing their contents over a period of days. Potential angiogenic proteins could be incorporated into the beads, and those that stimulated new blood vessel growth in the cornea could then be further purified and studied. This work was done in the mid-1970s with Robert Langer, Ph.D., now professor of Chemical and Biomedical Engineering at M.I.T., when he was a post-doctoral fellow in Folkman's lab at Children's Hospital. A third technique was the implantation of angiogenic proteins into chicken embryos cultured in dishes, where capillary growth could be easily observed.

By the late 1970s, Folkman's lab was working intensively to isolate and characterize an angiogenic protein from a tumor. Success came in 1983 when Michael Klagsbrun, Ph.D., and Yuen Shing, Ph.D., completely purified the first angiogenic protein from a tumor, basic fibroblast growth factor (bFGF). Other angiogenic proteins were subsequently discovered, including angiogenin in the laboratory of Bert Vallee, M.D., currently director of the Harvard Medical School Center for Biochemical and Biophysical Sciences and Medicine, and vascular permeability factor (VPF) was discovered in the lab of Harold Dvorak, M.D., Pathologist-in-Chief at Beth Israel Deaconess Medical Center and professor at Harvard Medical School.

Sixteen angiogenic proteins have now been discovered by labs in the United States and Europe; of these, bFGF is highly potent, and VEGF is the most specific stimulator of vascular endothelial cell proliferation. It's now known that tumor cells can trigger production of more than one kind of angiogenic protein, affecting different parts of the cellular signaling process leading to angiogenesis. Its also now known that these compounds are maintained on standby, releasable when angiogenesis is needed for bodily functions or when it is induced by a tumor.