Project Summary Directed evolution, which adopts principles of natural evolution to the laboratory, is singular in its impact on molecular engineering. As one example, it is responsible for the generation of the majority of approved therapeutic antibodies and those still in clinical development. Despite its progress, a valuable therapeutic niche remains outside its scope: the mammalian cell. While in vitro systems, phage, bacteria, and yeast have lent themselves to laboratory manipulation, mammalian cells have proven less tractable. Consequently, the power of evolution remains inaccessible to drug development pipelines that seek to modulate mammalian cell signaling. Further, many directed evolution campaigns result in biomolecules that fail in critical ways when transplanted to human cells. To address these limitations and advance methods in drug discovery, I will explore and mine viral diversity to create a novel system for molecular evolution in mammalian cells. Next, focusing on the 5-HT2A serotonin receptor, I will create extracellular nanobodies to template a drug discovery campaign via structural determination and in silico docking. The insights gained by these studies will be applied toward the directed evolution of state-specific nanobodies against dark GPCRs. This work will result in a general method for directed evolution in mammalian cells, chemical matter against HTR2A, and new paths forward for the deorphanization of GPCRs.