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B lymphocytes use somatic hypermutation (SHM) to optimize immunoglobulins. Although SHM can rescue single point mutations deliberately introduced into nonimmunoglobulin genes, such experiments do not show whether SHM can efficiently evolve challenging novel phenotypes requiring multiple unforeseeable mutations in nonantibody proteins. We have now iterated SHM over 23 rounds of fluorescence-activated cell sorting to create monomeric red fluorescent proteins with increased photostability and far-red emissions (e.g., 649 nm), surpassing the best efforts of structure-based design. SHM offers a strategy to evolve nonantibody proteins with desirable properties for which a high-through- put selection or viable single-cell screen can be devised. [ABSTRACT FROM AUTHOR]