Puffing of polytene chromosomes is associated with intense genetic activity. Genetic determinants of puffing in Drosophila can now be dissected by transplanting genes known to puff to other chromosomal locations by transposonmediated germ line transformation. Recent studies show that induction of a heat shock puff requires only about one hundred and fifty base pairs of DNA upstream of the gene, around the start of transcription, and not the protein coding sequence itself. Moreover, the size of the ectopic puff was found to be related to the size of the corresponding transcript. Similar work is now in progress with hormonally-controlled puffs.