These days, when it comes to meat alternatives, most eyes are on lab-grown and plant-based products. But one of the latest breakthroughs in sustainable, cruelty-free food comes from yet another kingdom of life: fungi.

The human-fungus bond has been cementing for thousands of years, during which our ancestors found countless ways to fit these nutritious organisms into their diet. Now, putting a distinctly modern twist on the relationship, scientists at UC Berkeley reported in Nature Communications earlier this year that they had genetically engineered meat-like patties out of mold.

And not just any mold — Aspergillus oryzae, also known as Koji, has been used in East Asia since prehistoric times to ferment starches into miso, sake, soy sauce, and other products. Vayu Hill-Maini, a postdoctoral researcher at UC Berkeley and lead author of the paper, sees his work as the logical 21st-century extension of that age-old domestication campaign.

“Humans have already tinkered with the genome and come up with this amazing species,” he says. “What I want to do is to say, let’s continue that evolution, but let’s bring in our contemporary tools to program it.”

Rewriting the Code

While genetic modification often involves transferring DNA from one organism to another, in this case Hill-Maini and his colleagues simply amplified Koji’s natural gifts. Using a gene-editing technology called CRISPR-Cas9, they altered the mold’s genome in two key ways, tapping into its latent potential.

First they ramped up production of the amino acid ergothioneine (sometimes called a “longevity vitamin”) to match that of oyster mushrooms, the best natural source for this potent antioxidant. Then, having boosted Koji’s nutritional profile, they set their sights on sensory appeal.

Meat gets its characteristic flavor, texture and hue from an iron-rich molecule called heme, which is already present in fungi at low levels. So the researchers modified Koji’s heme pathway until its mycelium turned red and its consistency became ideal for patty formation — thus a clump of tangled white roots transformed into a juicy burger, done medium rare.

Like all would-be purveyors of alternative meat, Hill-Maini knows that palatability is everything. “We can’t just program something and engineer it without actually thinking about deliciousness and the human experience,” he says. In particular, he plans to keep fine-tuning the texture of Koji 2.0, coaxing its filamentous cells into a better likeness of mammalian muscle fibers.

Read More: Are Fake Meat Products Good for You?

Sustainable Protein

The pitfalls of animal agriculture have led many innovators in search of substitutes — foods that can provide us the same protein and pleasure as meat, minus the widespread harm to our planet and public health.

Fungi aren’t newcomers to this scene. UK-based food company Quorn has been selling “mycoprotein” products for vegans and vegetarians since 1985, and in recent years the industry has seen a surge comparable — though on a smaller scale — to that of its lab-grown and plant-based cousins. The Berkeley paper is, however, the first example of using genetic modification to enhance those products.

Mycoprotein comes with some serious benefits: It doesn’t require expensive bioreactors like cultivated meat, it has a relatively benign carbon footprint, and it can sometimes even be manufactured from the waste products of other food operations. But Hill-Maini believes that bioengineering can make a good thing even better.

Although many fungi have a long history of human consumption, he says, “they’re not necessarily suited and evolved for the purpose of large-scale protein production.” The role of gene editing, then, “is in tuning that, and customizing it to be more tailored to our current needs as a society” — in other words, removing fungi’s inherent limitations as a mass-market food and enabling it to replace a greater percentage of meat intake.

Read More: Soon, You Could Be Wearing Mushroom Leather. But Will It Be Better for the Environment?

The Culinary Future of Engineered Fungi

For now that day is far off, and just how far no one can say. “This is an early prototype,” Hill-Maini says. “It’s not out there in the world.” Before it can be served, engineered Koji will need a green light from the FDA, certifying that it’s safe to eat.

There is precedent for genetically modified fungi: In 2016, a longer-lasting variant of the common white button mushroom received FDA approval. Still, it’s unclear what the regulatory process would be for Koji. And as with any GMO, consumer acceptance can be a challenge, though people are typically less wary when no genes have been exchanged between different organisms.

Whatever the practical future of his creation, Hill-Maini argues that the biggest advance here is actually conceptual. When it comes to complex editing in fungi, he says, “the tools are not that sophisticated.” Now that he and his team have built a CRISPR-Cas9 toolkit for one species, the next step is to devise a more comprehensive set of methods that can be systematically applied to a wider range of fungi.

As those technical resources grow, Hill-Maini hopes that scientists will delve deep into the culinary potential of genetically modified fungi, which is by no means limited to burgers. “The possibilities are really vast,” he says. “We started with a meat alternative because it makes sense from a planetary perspective, but I’m really excited about where else might this go.”

Read More: Should Eating Organ Meat Be the Next Foodie Trend?

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Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor’s degree in journalism and media production from Washington State University.