Doctors do not know the exact cause of glioblastoma. It develops from glial cells, which support nerve cells in the brain. Most often, glioblastoma begins as a grade IV tumor, with no prior evidence of a tumor. It sometimes can develop from lower-grade tumors. While the exact cause of GBM is unknown, people with genetic diseases like Turcot syndrome, Li-Fraumeni syndrome, and neurofibromatosis are at higher risk. GBM occurs in 2-3 out of every 100,000 American adults, according to the National Cancer Institute. It accounts for 17 percent of all brain tumors and is most common in adults between 45 and 70.