BACKGROUND: Male prevalence across histological subtypes of brain tumors is well known but unexplained. The effects of sex on human disease frequently result from acute actions of circulating sex hormones on cellular physiology and immune function. However, differences in brain tumor rates between males and females are comparable in prepubertal children, and pre and post-menopausal adults, suggesting that alternate mechanisms may underlie this disparity. Here we show that sex-specific differences exist in thresholds for malignant transformation within the astrocyte lineage and that this can contribute to male predominance in the most common brain tumor, glioblastoma (GBM). METHODS: Analysis of three publically accessible databases was utilized to determine whether sex differences are equally present in each of the four molecular subtypes of GBM. Stepwise transformation was modeled for Mesenchymal GBM in separate male and female murine astrocyte cultures by sequential inactivation of neurofibromin (NF1) and p53. Thresholds for transformation were measured in soft agar assays and with in vivo tumorigenesis assays. The molecular basis for differences in thresholds for transformation between male and female astrocytes was investigated in evaluations of stem cell induction, Rb inactivation and p21 activation. RESULTS: Significant male predominance was consistently observed in Proneural and Mesenchymal subtypes of GBM. We modeled Mesenchymal subtype GBM with combined loss of Nf1 and p53 function through cre-mediated knock-out of Nf1 (Nf1-/-) and expression of a dominant negative p53 construct (DNp53). These changes induced a subpopulation of cells with clonogenic, stem cell function in male but not female astrocytes. Male Nf1-/-,DNp53 astrocytes exhibited significantly greater growth compared to female Nf1-/-,DNp53 astrocytes. In addition, male but not female Nf1-/-,DNp53 astrocytes were transformed with either EGF treatment or in vivo implantation. Male Nf1-/-,DNp53 astrocytes exhibited greater in vivo tumorigencitiy regardless of the sex of the recipient mouse. Induced intracranial tumors were diagnosed as grade 4 astrocytomas based on standard histological features. Enhanced stem cell function and tumorigenicity in male Nf1-/-,DNp53 astrocytes was associated with greater inactivation of Rb and less p21 activation compared to female Nf1-/-,DNp53 astrocytes. CONCLUSIONS: Sex differences in cancer, including GBM can involve cell intrinsic sexual dimorphism in tumor suppressor pathways resulting in different thresholds for transformation in males and females. These results suggest that tumor suppressor and oncogene mechanisms should be evaluated in a sex-specific fashion and that screening and treatment of GBM and other cancers may need to be sex-specific. SECONDARY CATEGORY: n/a.