Yellow Fluorescent Protein (YFP) is a genetic mutant of green fluorescent protein, derived from Aequorea victoria. Its excitation peak is 514nm and its emission peak is 527nm. Like green fluorescent protein (GFP), it is a useful tool in cell and molecular biology, usually explored using fluorescence microscopy. Three improved versions of YFP are Citrine, Venus, and Ypet. They have reduced chloride sensitivity, faster maturation, and increased brightness (product of the extinction coefficient and quantum yield). Typically, yellow FPs serve as the acceptor for genetically-encoded FRET sensors of which the most likely donor FP is mCFP (monomeric cyan FP). The red-shift relative to GFP is caused by a Pi-Pi stacking interaction as a result of the T203Y mutation, which essentially increases the polarizability of the local chromophore environment as well as providing additional electron density into the chromophore. "Venus" contains a novel mutation –F46L– which accelerates the oxidation of the chromophore at 37°C, the rate limiting step of maturation. The protein has other mutations (F64L/ M153T/ V163A/ S175G), permitting Venus to fold well and giving it relative tolerance to acidosis and Cl.