Cystic fibrosis patient Els van der Heijden 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