Acromym for "Yellow Fluorescent Protein". It was derived through mutagenesis from the green fluroescent protein originally identified in the bioluminescent jellyfish Aequorea Victoria, and is currently used transgenically to visually label living cells in a large number of model organisms.

It should be noted that fluorescent proteins emitting in the yellow spectrum from other organisms (such as the jellyfish Phialidium sp. or the bacteria Vibrio fischeri) have also been called "YFP".