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