Fig. 3. Segment-specific functions are cis-regulatory regions. Three ventral nerve cords dissected out of Drosophila embryos immunostained for the Abd-B protein are shown. Parasegment (PS) borders and parasegment labels (PS10-PS13) are indicated according to the wild-type embryo. Below each cord is a diagram representing the state of each regulatory domain in PS10 through PS12, in which a black line represents a closed/silenced chomatin structure; a colored oval represents an open/active chromatin domain. (A) A wild-type embryonic ventral nerve cord has a distinct pattern of Abd-B expression that begins in PS10 and increases in each parasegment posteriorly. This is diagramed below as a parasegmentally regulated, sequential opening of chromatin domains. (B) The CNS from an iab-7^Sz mutant in which the entire iab-7 domain is absent. Its PS12 develops as a copy of PS11 (indicated by the similar staining pattern in PS11 and PS12). (C) A deletion that removes the Fab-7 boundary causes a fusion between iab-6 and iab-7, allowing the stronger iab-7 enhancers to become initiated by the iab-6 initiator element, resulting in an Abd-B expression pattern that is characteristic of PS12 being initiated in PS11, and a homeotic transformation of PS11 into PS12.