Addressing the challenge of protein engineering for human therapeutics, researchers have developed PROTEUS, a method that enables directed evolution of proteins within mammalian cells. Using the high-error replication competence of Semliki Forest Virus, PROTEUS introduces mutations efficiently, overcoming limitations observed with bacterial or yeast-based evolution systems. This approach has successfully optimized proteins with improved functionality specifically in mammalian environments, setting a new standard for protein-based drug development geared toward human use.