Red fluorescent protein （RFP） is a fluorophore that fluoresces red-orange when excited. Several variants have been developed using directed mutagenesis. The original was isolated from Discosoma, and named DsRed. Others are now available that fluoresce orange, red, and far-red. RFP is approximately 25.9 kDa. The excitation maximum is 558 nm, and the emission maximum is 583 nm. Genetic engineering has improved the utility of RFP by increasing the speed of fluorescent development and creating monomeric variants. Improved variants of RFP include mFruits （mCherry, mOrange, mRaspberry）, mKO, TagRFP, mKate, mRuby, FusionRed, mScarlet and DsRed-Express.