β-galactosidase, encoded by the lacZ gene in E. coli, is widely used as a reporter enzyme to study gene expression, protein-protein interaction and normalization of transfection efficiency in mammalian cells. The SensoLyteŽ ONPG β-Galactosidase Assay Kit utilizes the widely used chromogenic substrate, o-nitrophenyl β-D-galactopyranoside (ONPG) to determine the activity of β-galactosidase in transfected cells or tissues. In the presence of β-galactosidase, the colorless compound is hydrolyzed into o-nitrophenol, producing a yellow color that can be detected at absorbance 420 nm. The intensity of the color produced is proportional to β-galactosidase activity. The kit contains: b-galactosidase substrate, β-galactosidase standard, reaction buffer, stop buffer and optimized assay protocol. Assays are performed in a convenient 96-well format.