Tetsuro Fujita's eureka moment with a Himalayan fungus in 1985 may mean part of a $5 billion payout for Mitsubishi Tanabe Pharma Corp. a quarter-century later.

While the scientist drove over a bridge between Shikoku and Honshu on his way to take up a post researching traditional herbal remedies, Fujita realized that the fungus, used in a Chinese medicinal soup, must be suppressing the immune system of the insects on which it grew.

His research at Kyoto University not only helped yield Gilenya, a new treatment for multiple sclerosis — the debilitating condition afflicting more than 2 million people worldwide — but it also promised to bring Mitsubishi Tanabe its biggest money earner. Annual sales of the pill, the first for the autoimmune disease, may exceed $5 billion, UBS AG said.