StayGold-E138D is a monomeric green fluorescent protein with an excitation peak of 497nm and an emission peak of 505nm in vitro. It is an artificial derivative of the naturally occurring fluorescent protein encoded by the Cytaeis uchidae CU17S gene (GB:LC593677), derived by mutagenesis of StayGold, containing the E138D mutation (PMID:38081970).