4-Nitrophenyl 6-O-(b-D-galactopyranosyl)-b-D-galactopyranoside is a fluorescent substrate that is used in diagnostics and culture media. It can be used to detect the presence of aminoglycoside antibiotics, such as gentamicin, by using the fluorogenic or chromogenic reaction. 4NP6BG has been shown to bind to the beta subunit of RNA polymerase and inhibit transcription, which can be useful for testing environmental samples for the presence of aminoglycosides.