An organism is suitable for a GUS assay if it lacks naturally occurring β-glucuronidase activity or if the activity is very low (background activity). For this reason, the assay is not useful in most vertebrates and many molluscs. Since there is no detectable GUS activity in higher plants, mosses, algae, ferns, fungi and most bacteria, the assay is ideally suited for gene expression studies in these organisms, and considered the reporter gene of choice for in plant science.