fi-Galactosidase catalyzes the hydrolysis of various fi-galactosides. The colorimetric assay is based on the hydrolysis of o-nitrophenol-fi-D-galactopyranoside (ONPG) to o-nitrophenol and galactose. The absorbance produced by o-nitrophenol, a yellow color, is measured in a spectrophotom-eter. Although the colorimetric assay is not very sensitive, fi-galactosidase is often cotransfected with other plasmids and used to normalize for transfection efficiency between replicate transfections.
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