Reporter genes are fused to DNA fragments from a test gene or promoter. Reporter genes must be easily measured enzymatically or visually. Two common reporter genes are Β-galactosidase, green fluorescent protein (GFP) and yellow fluorescent protein (YFP). Β-gal is measured enzymatically using either X-gal (which Β-gal cleaves into a blue compound) or ONPG (which Β-gal cleaves into a yellow compound). GFP and YFP can be measured enzymatically, but are more easily measured visually because they fluoresce brightly.