"Directed evolution" is the process by which scientists produce tailor-made proteins for cell biology, physiology and biomedicine in the laboratory. Based on this method, Max Planck researchers from Martinsried have now developed a method to optimize proteins directly in mammalian cells. Using the new method, the scientists have produced the fluorescent protein mCRISPRred, which fluoresces brightly in lysosomes—very acidic, all-decomposing cell vesicles—which were previously difficult to label.