More than 50% of animals in the mutant cohorts GFAP-Cre; PdgfraK/+; INK4A/Arf−/− and Nestin-Cre; PdgfraJ/+; INK4A/Arf−/− harbored highly proliferative areas in the brain, reflecting early (Fig. 1c and S4a) and occasionally more advanced stages of brain tumor growth (Fig. 1d; Fig. S4b). This evidence concerns the gene GFAP and brain neoplasm.