Fig. 4. PS1' chromosomes from an animal containing the Sgs3-GAL4 driver and UAS-F645A responder. Chromosomes 2R (A) and 3L (B) from a white prepupa are shown. An identical pattern occurs in animals throughout the period from mid-instar transition to white prepupa. See text for discussion of the labeled sites. Note that these chromosomes are of a normal size for white prepupal polytene chromosomes, but that their puffing pattern is similar to that seen in the much smaller PS1' chromosomes of wild-type early third-instar larvae.