Injected mRNA expresses protein in living muscle

For the first time, researchers injected mRNA directly into the muscle tissue of live mice rather than cells in a dish, and the muscle cells produced the encoded protein. It confirmed the concept could work inside a living body, not just a lab dish. But two problems stood in the way of turning this into medicine: the mRNA broke down within hours, and the immune system often reacted to it as an invader, triggering inflammation. Solving both took another 15 years.