Scientists have recently made tremendous progress in treating cancers by activating the immune system to attack the tumor. However, these therapies are not effective against cancers with less DNA damage due to insufficient anti-tumor immune responses. The immune system is capable of attacking these tumors, but suppressive immune subtypes such as regulatory T cells (Tregs) are coopted by the tumor to protect itself. Tregs are associated with poor survival in many cancers, and show enrichment for particular T cell receptors (TCR). The TCR senses targets by binding to their peptide-MHC ligands, which display a cross-section of peptides expressed by a particular cell. Despite their important role in protecting tumors, the identity and specificity of tumor-resident Tregs is poorly studied. We are working to profile what T cells are enriched in low mutation rate cancers. We can then use approaches we have developed to find what these T cells as seeing in the tumor. This information will help us understand of one of the most important tumor-protective cell types, and may open the door to new cancer immunotherapies.