The most useful assay methods are those that produce a low-background signal and offer a wide range of detection, typically 10
-fold. Initially, radioactivity was used for this purpose; however, its purchase price and disposal costs, as well as its associated personal and environmental hazards, are prohibitively high. In addition, the safety training and maintenance of government-mandated records require extensive staff time and effort. These factors combine to make radioactive methods an alternative, but not the primary choice of detection technique. Luminescence detection, in contrast, is at least as sensitive as isotopic methods, is generally less expensive, and lacks the associated safety risks. Luminescence detection of a direct-labeled DNA probe is the most sensitive method of detection, eliminates interference from indirect labeling, is stable and reliable, and utilizes reagents with an extended shelf life (Table 1.1).