As the genes that regulate cell growth and proliferation are being identified and their encoded proteins characterized, our understanding of the molecular basis of tumor development is expanding rapidly. This review focuses on recent studies examining alterations of growth-regulatory genes in brain tumors, including those involving the inactivation of gene products that normally suppress cell growth (tumor-suppressor genes) or those involving the activation of gene products that promote cell growth (oncogenes) and presents information concerning the molecular biology associated with these changes.