The most formidable barrier in medicine is the one that separates the brain from the bloodstream. Some small, simple chemical compounds such as paracetamol can cross the blood–brain barrier, but for larger molecules, such as proteins and messenger RNA, it is completely impermeable. There is a good reason for this — toxins and pathogens swimming in the blood could wreak havoc in the brain. But the barrier also makes it difficult to treat the devastating diseases that affect this delicate organ.