Phenylgalactoside, oo sidoo kale loo yaqaan p-nitrophenyl β-D-galactopyranoside (pNPG), waa substrate synthetic oo inta badan loo isticmaalo tijaabooyinka bayoolojiga iyo molecular.Waxaa caadi ahaan loo isticmaalaa in lagu ogaado oo lagu cabbiro dhaqdhaqaaqa β-galactosidase enzyme.
Marka phenylgalactoside ay hydrolyzed by β-galactosidase, waxa ay sii daayaan p-nitrophenol, kaas oo ah kabo huruud ah.Xoraynta p-nitrophenol waxaa lagu qiyaasi karaa tiro ahaan iyadoo la adeegsanayo spectrophotometer, maadaama nuugista p-nitrophenol lagu ogaan karo mawjadaha dhererka 405 nm.