As a young man, he went through a life-changing period. Today, developmental biologist Jens Hjerling Leffler is trying to understand what happens in the brain as we grow into adulthood.
Text: Cecilia Odlind, first published in Medicinsk Vetenskap nr 3, 2026.
As a child, Jens Hjerling Leffler had two dreams: to become a researcher and to become very rich. He tried research first and chose the most difficult field he could find: neuroscience.
“It combines mathematics and physics with biology. It is incredibly complicated. I felt that this was something I wanted to immerse myself in,” he says, starting the coffee machine and putting in a pouch of snus, a Swedish oral tobacco product.
Early on, he recognised the need to systematically categorise the different cell types that make up the brain.
“At the time, I felt that many researchers were ignoring the complexity of the brain. It was seen as a mass of grey matter that researchers simply stuck electrodes into, without paying much attention to which cells they were actually recording from. But that matters a great deal,” says Jens Hjerling Leffler.
He spotted a niche and spent two of his five years as a postdoctoral researcher at New York University developing a way to isolate individual brain cells, an essential step towards characterising each cell type. His colleague Sten Linnarsson had developed a new method for measuring gene expression in individual cells. Together, they were able to characterise all the cell types in the brain in an atlas based on their gene expression at a given point in time.
“Today, that is the principle used by the vast majority of researchers,” he says.
In another project, the research group developed a method that uses gene expression to identify the cells underlying the risk of complex diseases. Over the years, genetic research has generated long lists of genes linked to an increased risk of mental illness.
“Hundreds of risk genes have been identified, but they do not actually tell us very much. With our method, which integrates genetics with neurobiology, we can instead link disease risk to a particular cell type, neural network or brain region. This significantly improves precision and makes the findings more useful in practice,” he explains.
Today, he is a member of the steering committee of a consortium that brings together 1,300 researchers in psychiatric genetics, where he leads a subgroup in the field known as functional genomics.
What does it mean to become an adult?
The question that has interested him all along is what it means to become an adult. More specifically, he wants to understand what happens during the later stages of brain development, when we make the transition from adolescence to adulthood. The frontal lobe is the last part of the brain to mature and is not fully developed until around the age of 25.
“This is a time when we have to shift from focusing on learning, finding ourselves and spending time with friends to planning, organising and taking responsibility. This later stage of brain development also coincides with an increased risk of several psychiatric disorders, including schizophrenia, bipolar disorder and depression. I want to understand more about how these things are connected,” he says.
Jens Hjerling Leffler himself went through a difficult and tumultuous period at around the age of 25, during which he also experienced depression.
“It reshaped me as a person. I went from being sociable and fairly easy-going, with a vivid imagination, to being a highly organised person who runs major projects. Something changed in my brain during that process, and something of who I was lost,” he says.
Studies brain development
Jens Hjerling Leffler has taken an interest in the molecular mechanisms involved in this developmental process. In experiments, the research group has been able to deactivate certain transcription factors at different stages of development in cells from mouse brains. They found that these factors control what a particular cell should be doing at a particular time. During early brain development, they determine which type of cell is formed and which region of the brain it should migrate to. Later, they control how it is integrated into a larger neural network.
“If we can gain a better understanding of these complex processes in brain development, I believe that it could also improve our understanding of psychiatric disorders,” he says.
But he does not expect to find any simple or definitive answers to his questions.
“No, we will not come to understand exactly what schizophrenia is. Our work will never be finished. For me, there is great freedom in studying something so extraordinarily complex and impossible to solve,” he says.
That does not mean that small advances in knowledge cannot make a major difference. Psychiatric disorders place a considerable burden on society and cause great suffering for many people. Knowledge about how to predict the risk of psychosis, for example, or how to distinguish between different types of depression could therefore be highly valuable.
“Even if we increase our knowledge only slightly, it could have a very substantial impact. At the same time, I believe that research without any obvious practical benefit is also important. Trying to understand the world is deeply human and has value in itself,” he says, now standing by the door and moving it back and forth to let some air into the room, which has become very warm.
The subject clearly matters to Jens Hjerling Leffler. He feels that researchers are sometimes misunderstood.
“Research is extremely difficult and involves a great deal of hard work. Most researchers sacrifice many other things in their lives,” he says.
Diagnosed with ADHD as an adult
Jens Hjerling Leffler lives with an ADHD diagnosis, which he received as an adult. He deals with the challenges that come with it every day.
“I regularly miss out on opportunities, and I can feel sad about that. My focus can easily shift, and some things are left unfinished, such as my undergraduate degree. But despite that, I managed to complete a PhD and become a professor,” he says.
Perhaps his somewhat unconventional background has contributed to his slightly rebellious idea that research should be like punk rock.
“What I mean is that everyone should be able to do research. All you need is a good question and the drive to answer it. I do not like it when people romanticise geniuses or show too much reverence for history and tradition, as that can stifle new ideas. What matters is thinking freely and daring to try new things,” says Jens Hjerling Leffler.
Excited about AI
However, deciding which new projects the research group should pursue in the future is not entirely straightforward, he says.
“AI is creating new possibilities, and everything is moving very quickly. Some of the research findings being published will probably turn out to be wrong. That makes it more difficult to decide how we should prioritise our work. But the new technology also presents enormous opportunities and will lead to a great deal of new knowledge. That excites me. I cannot wait to see what we will discover,” says Jens Hjerling Leffler.
And what became of his childhood dream of becoming very rich?
“I gave up on it. I realised how much I love being a researcher. You get to meet extraordinarily interesting people. You get to live in a world of ideas. Having the opportunity to think about how things work is a tremendous luxury,” he says.
Facts about Jens Hjerling Leffler
Name: Jens Hjerling Leffler
Title: Professor of Molecular Psychiatry at the Department of Medical Biochemistry and Biophysics, Karolinska Institutet
Age: 51
Family: Wife and two children, aged eleven and six
I enjoy: Driving fast, sailing and birdwatching. But I rarely have time to do any of them. I mostly spend time with my family, which I love.
Motto: Do I have to choose one? Out with the old, in with the new.
Jens Hjerling Leffler on…
…what is on his mind at the moment:
How does a cell know how old it is? It was long thought that a cell’s age was determined by hormones, but sexual maturation does not correlate with brain development.
…being on stage:
It makes me come alive. I do not get nervous at all. Because I think while I speak, I sometimes even surprise myself with new insights during a presentation.
…decision week:
For a period in my life, I took one week off every year to walk in the forest and think about my future. It was a way of finding direction and avoiding overthinking things for the rest of the year.
…social networks:
Research happens when people come together. The best thing about the lab is the people in it. A friend has described me as a “high-connectivity node”, like a hub connecting neurons in the brain.
