E2-Orange is a tetrameric orange fluorescent protein with an excitation peak of 540nm and an emission peak of 561nm in vitro. It is an artificial derivative of the naturally occurring fluorescent protein encoded by the Discosoma drFP583 gene (GenBank:AF168419; AAF03369), derived by mutagenesis of DsRed-Express2, containing the mutations Q66T, V71A and S179T relative to DsRed-Express2. E2-Orange matures quickly and has low cytotoxicity (PMID:19344508).