Scientists studied how different fungal species produce mycophenolic acid, a drug used to prevent transplant rejection in millions of patients worldwide. By analyzing the genomes of many fungal species, they found that only a few fungi can make this important drug, and they discovered that these fungi have different ways of protecting themselves from being poisoned by their own medicine. This research helps us understand how fungi evolve to produce valuable medicines and could lead to better ways to produce immunosuppressants.