X-Gluc (5-bromo-4-chloro-3-indolyl-β-D-glucuronic acid) is a substrate for ß-glucuronidase (GUS), which is encoded by gusA, a widely used reporter gene. The glucuronidase cleaves X-Gluc to produce colorless glucuronic acid and an intense blue precipitate of chloro-bromoindigo.

X-Gluc is used for the histochemical detection of GUS expression in plant cells and tissues, detection of infections caused by E.coli and detection of bacterial contamination in food and water samples.