When immune cells and developing tumor cells localize to a common microenvironment, an assemblage of interactions takes place; this results in either tumor destruction by way of immunosurveillance or tumor outgrowth. These events put a functional imprint onto the emerging tumor repertoire because tumor cells arising in the presence of a fully functional immune system are less immunogenic than those that develop in the absence of immunity (i.e. in RAG2-/- and perforin-/- mice). However, other studies suggest that the immune system can also actively promote formation of certain tumors. These apparent disparate effects of immunity on tumorigenesis provide a unique model for study of the decision-making process that dictates immune function within a tumor.