Yoon Seok Kim, Flying In Orbit Of Nobels

"For a lot of researchers, the Nobel Prizes are like the Oscars. On Monday, I was watching the stream live. When they announced that Karl had won the Prize in Physiology or Medicine, I literally yelled out for joy."

In his lab at EPFL, a few days after the news, Yoon Seok Kim is still a little giddy. Karl Deisseroth, the Stanford neuroscientist and practicing psychiatrist who shares this year's Prize with Peter Hegemann (Humboldt University Berlin) and Georg Nagel (University of Würzburg), was his thesis supervisor and then mentor over nine years of close collaboration in California. Before hearing the announcement, Kim, an Assistant Professor of Life Science Engineering, had an inkling. "Karl was always on the shortlist. I knew it was only a matter of time, but I had no idea it would happen this year. It was such a wonderful surprise."

Within moments, he had texted both Deisseroth and his wife Michelle Monje, a Stanford neuro-oncologist who also happens to have been Kim's supervisor on his postdoctoral research, to offer congratulations.

The rise of optogenetics

The 2026 Nobel Prize honors the discovery of light-gated ion channels and the birth of optogenetics. This technique, which Kim himself helped push forward, has been a game changer for neuroscience. "Light-gated ion channels were originally discovered in single-cell algae," he explains. "Plants respond to light - that's what they do - but how they do it had been a mystery. Peter [Hegemann] and Georg [Nagel] took a close look into those algae and found this really interesting protein, channelrhodopsin, that actually converts light energy into electricity. They are a big part of the discovery. Karl was the person who actually put that into neurons."

This is what makes optogenetics: give chosen neurons the gene for this algal protein, and a flash of light becomes enough to switch them on. For the first time, researchers could test what a brain circuit actually does, rather than merely observe it. "It was like the PCR of neuroscience," says Kim, referring to the DNA-copying technique found in every biology lab. "You just plug it in, and it works!"

Remarkably, Deisseroth is not the first Nobel laureate Kim has worked with. His trajectory has run through the labs of three scientists honored by the prize.

At the start of his scientific journey, in 2007, Kim came to Stanford as a biology undergraduate. After a two-year break in the military (a compulsory service in South Korea), he returned and joined Thomas Südhof's lab in July 2013, to work on neuroligin, a synaptic protein linked to autism. The subject was personal, his older brother being on the autism spectrum (read more about Kim's life and work here). Three months later, Südhof won the Nobel Prize in Physiology or Medicine for his work on how neurons communicate.

In his lab, Kim learned molecular biology and biochemistry, and found his calling. "I realized that I loved research and wanted to go deeper into the molecular logic of the brain."

The following year, he began his PhD. After Südhof, he chose to do his next rotation with Brian Kobilka, 2012 Nobel laureate in Chemistry for solving the structure of G protein-coupled receptors. Kim had never taken a biochemistry class. "With Brian, none of the protocols are commercial. Every line is a culmination of thirty years of hard work and troubleshooting. Knowing how to purify one protein doesn't mean the same method will work on the next one." Every day was a challenge. And, at the start, results were scarce. "But what doesn't kill you makes you stronger, and I think that really made me very strong."

Next came a bold move. The young grad student decided he wanted to solve the atomic structures of channelrhodopsins. The proteins were already widely used to study the brain, yet only one structure had ever been solved, and there was much to learn about how they worked and how to make them even more useful. He proposed a co-advisorship between Karl Deisseroth and Brian Kobilka. "It's pretty unusual for a student to take on such a risky project on their own, but I was so into the idea that I just had to try. Standing on their shoulders, I thought I could take on such challenges."

Deisseroth, who at that point was running a large laboratory as well as a clinic, while raising four young children (he has five today), agreed to meet immediately. They talked over coffee, and the project was on.

A hundred thousand attempts

It was, however, no walk in the park. Proteins measure just a few nanometers, too small to appear under ordinary microscopes. To see them, researchers coax millions of copies into a crystal, then fire X-rays at it. "You get these diffraction patterns, which are like a shadow image of the actual atoms," Kim says. "From the shadows, you can infer how they look." Membrane proteins, wrapped in a fatty envelope, are notoriously reluctant to crystallize, and the process is "almost completely empirical". So he tried: some 200 engineered versions of the proteins, more than 100,000 crystallization trials in all.

At Stanford, his crystallization plates ended up filling an entire communal room, until his colleagues told him to clear them out. He checked them one last time. "I went through all of them. And then this little guy showed up…" The crystal that would change everything had needed about a month to grow. Under the X-rays, it produced the first sharp diffraction pattern. In the summer of 2017 came what Kim calls "the eureka moment": the first structure. He emailed it to Deisseroth, who was on holiday. "He rang back almost immediately. I can still vividly remember the moment."

The results appeared back to back in Nature in 2018, followed by four more papers in Science and Cell, all with Kim as first author. For the first time, researchers could see how channelrhodopsins capture light and let charged particles through, and use these blueprints to design better tools. In Deisseroth's lab, a team of some 40 researchers, this line of work was largely Kim's. "Karl is one of the most brilliant, kind, courageous, and generous people I've ever known, and I am deeply grateful for everything he has given me. I'm thrilled that we were able to uncover how these proteins work and use that knowledge to develop better tools for exploring one of biology's greatest mysteries: how the brain works."

Beyond light

Today, Deisseroth's team is moving optogenetics towards clinical applications. Kim, meanwhile, has struck out on his own. After his postdoctoral work, he crossed the Atlantic in 2025 to join EPFL, where he leads the Laboratory of Molecular and Synthetic Neuroengineering. Currently a team of six, it will be welcoming two more researchers in the coming months.

Convinced these tools offer "unlimited possibilities", Kim is now looking beyond light. His lab also studies proteins that convert mechanical forces into electrical signals, investigating how cells respond to their surroundings. In collaboration with EPFL's protein-design groups, he is also engineering proteins with new functions. His work extends into cancer neuroscience, exploring how brain tumors communicate with neurons and how mechanosensitive signaling and light-gated tools might help scientists understand - and ultimately disrupt - these interactions.

Kim and Deisseroth have remained close. Over one last meal before Kim left Stanford, Deisseroth gave him a piece of advice: "Just do the work you find most beautiful. Everything else will work itself out." Kim has his own reading of it. "Many people decide their career path based on societal or peer pressure. These things are transient. You can only be successful when you work on what makes you happy, things you really believe in."