Notes: p-Nitrophenyl β-D-galactopyranoside is a chromogenic substrate for β-D-galactosidase.{64531} Upon enzymatic cleavage by β-D-galactosidase, p-nitrophenol is released, which can be quantified by colorimetric detection at 420 nm as a measure of β-D-galactosidase activity. p-Nitrophenyl β-D-galactopyranoside has been used to determine the substrate specificity of T. maritima MSB8 β-galactosidase.{64532}