mRuby is a monomeric red fluorescent protein with an excitation peak of 558nm, an emission peak of 605nm and a large Stokes shift of 47 nm. It is an artificial derivative of the naturally occurring fluorescent protein encoded by the Entacmaea quadricolor eqFP611 gene; in addition to mutations related to improving its use as a fluorescent tool, the C-terminus has been mutated to remove a naturally occurring peroxisomal targeting signal found in the eqFP611 protein. The properties of mRuby make it useful for imaging, particularly fluorescence resonance energy transfer (FRET) applications (PMID:19194514).