Biologists are constantly seeking more sensitive assays to detect the presence of organisms or telltale DNA, RNA, and proteins. Although radioactive tags incorporated into the target itself (or into a complementary strand)-and later detected by Geiger counters or film exposures-have traditionally given good sensitivity, the problems of waste disposal and laboratory monitoring have driven a search for alternative tags that have radioactivity's sensitivity but avoid its hazards.

Fluorescent tags-bound directly to the analyte-are easily disposable, but these reagents are not always sensitive enough to meet a researcher's needs (J. Kling, The Scientist, April 14, 1997, page 14). As a result, radiolabels continue to be a mainstay in many smaller research laboratories.

But where fluorescent tags fail, chemiluminescence and bioluminescence detection can often step in. These systems rely on a reporter enzyme-which signals the presence of the target and produces the luminescence-and a substrate, usually added separately. The...

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