The LacZ gene from E. coli is one of the most commonly used reporter genes for testing the efficiency of expression vector mediated gene transfer and for studying the regulation of promoters of genes. The LacZ gene encodes the enzyme β-galactosidase, which is very stable, resistant to proteolytic degradation, can utilize a variety of substrates and can be easily assayed in situ. The β-Galactosidase staining kit utilizes X-gal as the substrate.