Malignant astrocytomas are common human primary brain tumors that result from neoplastic transformation of astroglia or their progenitors. Here we show that deregulation of the c-Myc pathway in developing astroglia predisposes mice to malignant astrocytomas within 2-3 weeks of age. The genetically engineered murine (GEM) gliomas harbor a molecular signature resembling that of human primary glioblastoma multiforme, including up-regulation of epidermal growth factor receptor and Mdm2. The GEM gliomas seem to originate in an abnormal population of glial fibrillary acidic protein-expressing cells in the ventricular zone and, analogous to human glioblastomas, exhibit molecular and morphological heterogeneity. Levels of connexin 43 in the majority of the tumors are unaltered from normal tissue, indicating that GEM tumors have retained the capacity to establish syncytial networks. In line with this, individual glioma foci are composed of a mixture of actively proliferating cells expressing c-Myc and proliferating cell nuclear antigen and less dividing bystander cells that express glial fibrillary acidic protein and the broad complex tramtrack bric-a-brac/poxvirus and zinc finger domain protein HOF. A subset of the transgenic mice harbored, in addition to brain tumors, vestigial cerebellums in which granule cell migration and radial Bergman glial cell differentiation were disturbed. These observations argue for a window of vulnerability during astrocyte development where c-Myc overexpression is sufficient to trigger the neoplastic process, presumably by inducing the sustained growth of early astroglial cells. This is in contrast to most other transgenic studies in which c-Myc overexpression requires co-operating transgenes for rapid tumor induction.