Our brain comprises 85 billion nerve cells and just as many so-called glial cells, which work in close contact with the former to guarantee their proper function. All originate from brain stem cells. But what decides when and how many of them become neurons or glial cells? A new study led by the Laboratory of Cerebral Cortex Development of SISSA has shown how the Foxg1 gene, already involved in numerous processes of cerebral development and in rare disorders like Rett and West syndromes, plays a fundamental role in piloting the differentiation of stem cells, guaranteeing that neurons and glial cells are produced in the right quantity and at the right moment. The work, published in Cerebral Cortex and conducted in collaboration with the University of Cambridge and the IRCCS Burlo Garofolo, opens new roads to understanding and treating incurable genetic diseases.