Green Fluorescent Protein (GFP) is a naturally fluorescent protein isolated from the jellyfish Aequorea victoria. GFP is a fluorescent marker that makes possible the visualisation of protein in the cell. Cells can be stained by them and examined by fluorescence microscopy to reveal the location of a protein of interest. For example, glucocorticoid receptor protein is a transcription factor that controls gene expression in response to the steroid hormone cortisone. The receptor is linked to the GFP. Fluorescence microscopy reveals that, in the absence of the hormone, the receptor is located in the cytoplasm and in its presence, the receptor is translocated to the nucleus, where it binds with the DNA.