(Science: molecular biology) Expanded areas of a polytene chromosome. at these areas the chromatin becomes less condensed and the fibres unwind, though they remain continuous with the fibres in the chromosome axis.
A puff usually involves unwinding at a single band, though they can include many bands as in balbiani rings. Puffs represent sites of active rNA transcription. The pattern of puffing observed in the larvae of drosophila, in different cells and at different times in development provides possibly the best evidence that differentiation is controlled at the level of transcription.