2-Nitrophenyl-beta-D-thiogalactopyranoside is a chromogenic substrate used to detect the presence of galactosidase enzymes in various biological and biochemical assays. It consists of a nitrophenyl group attached to a galactose sugar molecule through a thioglycosidic bond. When acted upon by enzymes such as galactosidases, it releases the colored nitrophenyl group, which can be detected using spectrophotometry. This substrate is commonly used in assays for the detection of LacZ activity in bacterial cultures, as lacZ encodes for beta-galactosidase.