This segment illustrates how Dr. Judah Folkman designed experiments to provide solid proof of angiogenesis' existence, and the further identification of angiogenesis factors. The identification of angiogenesis factors was so time consuming (it took several years of effort) that even Dr. Folkman doubt whether this factor existed. The quote from Dr. Folkman: "In research, there's a very fine line between persistence and obstinacy. You do not know whether you're persistent a little while longer, you'll make it, or whether you're just being obstinate; it doesn't exist. And of course you can keep on going--stay with an idea too long, called pigheadedness." This is Part 3 of a 8 part series (http://www.pbs.org/wgbh/nova/cancer/program.html) that introduces a possible non-toxic cancer treatment and highlights the long journey of how angiogenesis (new blood vessel growth) was discovered, how most scientists thought angiogenesis as ridiculous, how angiogenesis was proven existed and crucial to tumor growth, and the effort of searching angiogenesis inhibitors. This series not only shows scientists' problem-solving in research laboratory, but also shows how scientists face the harsh critics/examines from their peers. Moreover, this series shows even though the logic of an experimental design could be very simple, it often takes many years of hard work to identify the target of interest. Biotechnologies such as protein purification and cell cultures are shown in this series.