You might think that Joseph Schlessinger — chair of the department of pharmacology at Yale, pioneer in cancer research and one of the top 30 most-cited scientists of the 1990s — went through life with a plan.

“Nonsense!” Schlessinger said, denying several times in his talk on Thursday that his career was anything more than a “set of interesting accidents” guided by curiosity.

Schlessinger was on campus for the Ef Racker Lecture, a yearly biology and medicine talk named to commemorate one of the scientists who helped found the biochemistry department at Cornell. The lecture series began in 1992 with James D. Watson as the inaugural speaker and annually features a notable scientist from a field of study related to the late Ef Racker. Schlessinger, who worked briefly with Racker as a postdoctoral associate at Cornell from 1976-1978, has spanned the disciplines from chemistry and physics to biology and immunology.

Born in the Nazi-occupied region that would become Croatia, Schlessinger immigrated with his family to Israel in 1948, where he later served in the infantry brigade for both the Six Day’s War and the Yom Kippur War. After this, Schlessinger went to study in a diverse range of institutions, from Hebrew University in Jerusalem to the National Institutes of Health.

One of Schlessinger’s biggest breakthroughs came in the 1980s when he and his colleagues discovered a link between epidermal growth factor (EGF) and cancer in chickens. EGF is a naturally occurring growth factor involved in the regulation of cell growth, proliferation and differentiation. Schlessinger identified EGF as an oncogene, meaning that when it is mutated or expressed at an abnormally high level, it can cause cancer. Now, several cancer drugs have been developed to target EGF and limit its over-expression. Although response from the scientific community was enthusiastic, Schlessinger still remembered one member of the media who asked him, “Why are you trying to cure chicken cancer?”

After this discovery, Schlessinger determined to pursue the matter more deeply. “I was not satisfied with finding that these molecules were linked to cancer,” he said. “I wanted to understand how they worked.”

Schlessinger continued to develop his theories about EGF, focusing its receptor. The receptor is turned “on” when a phosphate is added in a process called phosphorylation. Schlessinger’s research led him to believe the receptor was self-phosphorylating, meaning that it added a phosphate through its own enzymatic activity. A novel idea at the time, Schlessinger’s theory added to what he referred to as a “violent debate” about how the receptor worked. It took five years of rejection before his paper on the topic was eventually published.

In 1991 Schlessinger made his debut in the business world by co-founding SUGEN, a biopharmaceutical company that Schlessinger called the first company whose goal was to develop new drugs for the treatment of cancers and other similar dysfunctions. SUGEN went on to develop a drug that would become the first to be approved by the FDA for two different types of cancer. Initially baffled by the world of big business, Schlessinger recalled that he learned two things from the experience. The first, he said half-jokingly, was that “it’s nice to make a lot of money,” and the second was that “it’s very difficult to be innovative in a large company.”

But Schlessinger was not done with business. Frustrated with the entrenched methods of the industry, he remembered thinking, “What the heck, why don’t I found another company?” In December 2000 this whim became reality, resulting in his second company: Plexxikon. This time Schlessinger installed himself as chairman of the board so that he would have more freedom to experiment.

Currently, Plexxikon is developing a drug for the uncommon but deadly skin cancer melanoma. Although the drug has only passed Phase One of testing, Schlessinger reported that 70 percent of subjects responded well — a rare occurrence at such an early stage in testing. Because of this, the drug is now on a fast track to approval by the FDA.

Schlessinger founded yet another pharmaceutical company in 2008 called Kolltan. He described each new company as a learning experience. “Hopefully, in my tenth company, I will make no mistakes,” he said, laughing.

The lecture concluded with a message to the students about following curiosity and taking an interest in new problems. “Do me a favor,” Schlessinger said. “Never be practical, especially when you are young.”