Recent evidence has shown that many different tissues accumulate mutations even though the tissue is phenotypically normal. Therefore, generating mouse models for visualizing the tissue level effects that happen after oncogenic mutation in a single, isolated cell are critical for understanding tumor initiation and the role of competition in stem cell dynamics. Most mouse models have oncogenic mutations at the level of the entire mouse, the entire tissue, or all cells of a specific type in a tissue. However, these mouse models do not mimic the microenvironmental interactions that occur after an isolated cell acquires an oncogenic mutation because of the large number of mutant cells. We developed a mouse model for sporadic and isolated mutation of target alleles to better address the questions of sporadic cancer and stem cell competition. The following chapter describes methods for utilizing this mouse model and a few examples of the novel findings of using such a model.