Fig. 4. Reporter constructs identify initiator and maintenance elements. Drosophila embryos immunostained for the ß-galactosidase protein. (A,C) Early embryos at germband extension, where the posterior parasegments have curved around towards the dorsal side; (B,D) later stage embryos. (A,B) Embryos in which lacZ expression is driven by an element from the iab-6 region. (A) In early embryos, lacZ expression is restricted to the posterior of the embryo, with its anterior border positioned at PS11. (B) At later stages of development, the repression of lacZ anterior to PS11 is lost, as the iab-6 element becomes active throughout the embryo. (C,D) Embryos in which lacZ expression is driven by a DNA fragment derived from iab-5. (C) In early embryos, the anterior border of lacZ expression is positioned at PS10. (D) Later in development, the anterior border of lacZ expression is maintained, indicating the presence of both an initiator and a maintenance element on this fragment. ant., anterior; post., posterior.