Engineered proteins with enhanced or altered functionality, generated for example by mutation or domain fusion, are at the core of nearly all synthetic biology endeavors in the context of precision medicine, also known as personalized medicine. From designer receptors sensing elevated blood markers to effectors rerouting signaling pathways to synthetic transcription factors and the customized therapeutics they regulate, engineered proteins play a crucial role at every step of novel therapeutic approaches using synthetic biology. Here, we discuss recent developments in protein engineering, aided by advances in directed evolution, de novo design and machine learning. Building on clinical successes already achieved with chimeric antigen receptor (CAR-) T cells and other cellbased therapies, these developments are expected to further enhance the capabilities of mammalian synthetic biology in biomedical and other applications.