used to determine whether a given fragment of DNA
has a promoter
on it to encourage transcription
to occur, by attaching the gene
(called the CAT gene
) which codes for the CAT enzyme
to it, and observing whether the CAT enzyme is produced.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary. For copyright details, see the BioTech homenode.