It only takes one.

Scientists have discovered, as shown here in a recent article, that a single gene defect causes a brain tumor. A defect in the BRAF gene alone is the cause for the majority of pilocytic astrocytomas, the most common brain tumor in children. Typically, this tumor is slow-growing and benign but it is difficult to surgically remove all of it, requiring patients to go under chemotherapy which can lead to many severe side-effects. The defect in the BRAF gene causes a cellular signaling pathway to be permanently activated, when in a healthy cell should only be activated in case of acute need. By adding a defective BRAF gene into a virus and injecting it into the neuronal precursor cells of mice, Jan Gronych and colleges of Heidelberg University Hospitals found that in 91% of the animals injected, tumors formed around the site of injection. Cells of the tumors were examined, and they showed typical symptoms of defective BRAF genes, a permanently activated MAP kinase enzyme, which constantly transmits growth signals in cancer cells. This researched showed that it only takes one defective gene to cause pilocytic astrocytoma.