Our lab investigates how a cancerous tumor forms in its earliest stages so that tumors can be more readily identified and treated. Tumor suppressor genes encode proteins that normally keep cell growth and division in check, preventing excess cell proliferation and the formation of tumors. Some forms of cancer are initiated when both copies of tumor suppressor genes are lost in single or a few cells, and the cells begin to divide unchecked. The most efficacious time to stop the division of these mutated cells would be right at the point of the initial tumor suppressor gene loss; however it is nearly impossible to identify these cells. Previously, we developed and tested a mouse genetic model system that sporadically shuts down tumor suppressor genes in cells and labels the resulting mutant cells so that they can be visualized within hours of their formation. Using this system, we study what role a specific tumor suppressor, p53 plays in the initiation of brain tumors.