Scientists already know that schizophrenia has genetic roots, but the condition has still been something of a mystery. How, exactly, does it get started? Researchers at Harvard and MIT now have a better idea. Thanks to both lab testing and a 100,000-person sample of DNA, they've determined that an immune system gene (complement component 4, aka C4) can play an important role in the disease. If a variant of C4 is too active in pruning brain synapses (severing links between neurons) during adolescence, the risk of developing schizophrenia goes up -- it's breaking connections at a key point in development.