Here's the problem. You're a recently conceived embryo, barely more than ablob of nearly identical cells. But it's time to start divvying up the physiological chores required of a developing organism. And somebody's got to sign up for brain duty. Who gets the job?
The process is a mystery, but researchers are a step closer to solving it. They've identified a protein, called NRSF (for neuron-restrictive silencer factor), that plays cellular guidance counselor. When NRSF show up, one of a cell's possible career paths--becoming a brain cell--disappears.
The protein does its job in a blunt but effective manner. Building a brain cell requires particular genes useful only for neurons. But since each of our body's cells contains the same DNA, cells destined not to become neurons need a way to turn off those neuron-specific genes.