Across most of our brain, the neurons that we’re born with are the ones we have throughout our lives. We make new ones in two small areas—the hippocampus, involved in memory and navigation, and (briefly in) the olfactory bulb, responsible for smell. Elsewhere, we’re stuck with the same old supply. That’s partly why diseases like Alzheimer’s and Parkinson’s are so devastating. They destroy neurons that won’t get replaced… at least not without a little help.

Most of our cells are stuck in their ways, so a neuron will always be a neuron and a skin cell will always be a skin cell. But with the right combination of molecules, we can nudge them into a mid-life career change. This research on