Directed evolution has yielded myriad molecules to transform biotechnology and industrial processes. Biomolecule discovery required to fuel therapeutics, however, lies beyond the scope of in vitro and prokaryotic directed evolution, which frequently results in products that fail to function properly in mammalian systems. Motivated by the ever-growing demand for biomolecules tailored to probe and perturb mammalian biology, my laboratory is developing a novel mammalian directed evolution platform. We propose to break through the technical bottleneck using CRISPR-guided in situ gene diversification. Our directed evolution platform is designed for facile customization and adaptation, similar to the CRISPR interference and activation libraries widely used for genetic screens. Our goal is to fuel therapeutics discovery by placing mammalian directed evolution within reach of all labs with general molecular and cellular biology expertise.