A reporter gene is a gene coding a protein molecule that can be easily assayed. The main use of promoter gene is to show whether a promoter the gene is linked to, is activated or not. A very common reporter gene is [[Β-galactosidase|beta-galactosidase]. In experiments within the lab, reporter genes are usually ligated into a plasmid vector which contains a gene to select against the bacteria that have not taken up the plasmid (such as antibiotic resistance). The DNA fragments which have different length sequences of DNA upstream of the transcription start site.
The plasmids are then separately transfected into cells, such as cultured E. coli. The cell extract can then be easily assayed to show levels of the reporter enzyme and therefore show how active the promoter is.