CRISPR Lights up In Situ Protein Evolution.

In this issue of Cell Chemical Biology, Erdogan et al. (2020) describe a new CRISPR/Cas9-based strategy for performing directed evolution of mammalian proteins in situ. Using this technique to select functional mRuby3 variants within lysosomes, they identify mCRISPRed, a fluorescent protein that displays robust stability and activity at low pH.