UTRECHT, Netherlands (AP) — Els van der Heijden, who has cystic fibrosis, was finding it ever harder to breathe as her lungs filled with thick, sticky mucus. Despite taking more than a dozen pills and inhalers a day, the 53-year-old had to stop working and scale back doing the thing she loved best, horseback riding.

Doctors saw no sense in trying an expensive new drug because it hasn’t been proven to work in people with the rare type of cystic fibrosis that van der Heijden had.

Instead, they scraped a few cells from van der Heijden and used them to grow a mini version of her large intestine in a petri dish. When van der Heijden’s “mini gut” responded to treatment, doctors knew it would help her too.

“I really felt, physically, like a different person,” van der Heijden said after taking a drug — and getting back in the saddle.

This experiment to help people with rare forms of cystic fibrosis in the Netherlands aims to grow mini intestines for every Dutch patient with the disease to figure out, in part, what treatment might work for them. It’s an early application of a technique now being worked on in labs all over the world, as researchers learn to grow organs outside of the body for treatment — and maybe someday for transplants.

So far, doctors have grown mini guts — just the size of a pencil point — for 450 of the Netherlands’ roughly 1,500 cystic fibrosis patients.

“The mini guts are small, but they are complete,” said Dr. Hans Clevers of the Hubrecht Institute, who pioneered the technique. Except for muscles and blood vessels, the tiny organs “have everything you would expect to see in a real gut, only on a really small scale.”

These so-called organoids mimic features of full-size organs, but don’t function the same way. Although many of the tiny replicas are closer to undeveloped organs found in an embryo than adult ones, they are helping scientists unravel how organs mature and providing clues on how certain diseases might be treated.

In Australia, mini kidneys are being grown that could be used to test drugs. Researchers in the U.S. are experimenting with tiny bits of livers that might be used to boost failing organs. At Cambridge University in England, scientists have created hundreds of mini brains to study how neurons form and better understand disorders like autism. During the height of the Zika epidemic last year, mini brains were used to show the virus causes malformed brains in babies.

In the Netherlands, the mini guts are used as a stand-in for cystic fibrosis patients to see if those with rare mutations might benefit from a number of pricey drugs, including Orkambi. Made by Vertex Pharmaceuticals, Orkambi costs about 100,000 euros per patient every year in some parts of Europe, and it’s more than double that in the U.S., which approved the drug in 2015. Despite being initially rejected by the Dutch government for being too expensive, negotiations with Vertex were reopened in July.