Mikitani’s father’s pancreatic cancer diagnosis (and eventual death) spurred the serial entrepreneur to form a biotechnology outfit centered on “photoimmunotherapy.” In simpler terms, Rakuten Aspyrian is attempting to treat cancer with literal light.

That journey began on the heels of the elder Mikitani’s diagnosis. “One of my friends called me, who was also a friend of my father’s… and said, by the way, my cousin is doing this new project, trying to cure cancer with light,” Mikitani explained during a one-on-one interview at the Fortune Brainstorm Health conference on Wednesday. “I was like, this must be a joke. But I was desperate.”

So how does the technology work? It involves infusing cancer patients with something called an “antibody-drug conjugate.” (that’s a fancy way of saying antibodies which can bind to specific spots in the body, such as tumors) that are connected to cancer-killing drugs. Next, the photoimmunotherapy process uses near-infrared light, transmitted via catheters or other methods, to initiate rapid cancer-killing activity at the tumor site.

The company’s photoimmunotherapy treatments are currently in late-stage clinical trials for head and neck cancers, with earlier stage studies in other kinds of cancers planned for the near future. “We have seen some [positive] cases which were very difficult to see with the traditional approach,” said Mikitani.

With so many kinds of more-established approaches to cancer treatment on the market, why would Mikitani choose this route? “I was ignorant enough to bet on this one,” he explained.

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