Giant puffs on a polytene chromosome in the salivary glands of certain insect larvae which, when present, means that transcription of DNA is taking place. The puffs are areas where the chromosome has become decondensed for the purpose, and are easily visible with a light microscope.

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From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.