The immune system plays a major role in safeguarding us from cancer. Tumor progression is closely linked to functional suppression of T cell responses, and strategies to reactivate tumor-specific CD8+ T cells hold great promise for cancer therapy, as evidenced by recent clinical breakthroughs. However, the further exploration of such therapies is hampered by our incomplete understanding of key genes and pathways involved in suppression of anti-tumor T cell immunity. Here, we propose an innovative approach combining three well-established experimental systems - genetically engineered mouse models of human cancer, T cell receptor/cognate antigen transgenic mice, and advanced in-vivo RNAi screening technologies - to systematically identify and functionally evaluate genes involved in this process. Specifically, we will establish an experimentally scalable in-vivo RNAi system to investigate genes modulating interactions between OT-I transgenic T cells and cOVA expressing cancer models, and use it in a multiplex in-vivo RNAi screen to survey a focused shRNA library targeting ~400 candidate T cell suppressor genes. Using this innovative screening approach and a sequential functional validation strategy, we seek to identify and functionally study new factors involved in the suppression of anti-tumor T cell immunity, ultimately to guide the development of more effective targeted cancer therapies.