Fig. 9. Model for D-h regulation. Ci acts through the HHRE to activate expression in a broad AP boundary stripe that can be divided into zones 1 (blue) and 2 (red). The intact D-h element directs expression only in zone 1. Brk and perhaps a second factor, X (see text), act through the CMB to repress D-h expression in zone 2. In zone 1, Dpp signaling prevents Brk repression of D-h expression. Dpp function might be restricted to repression of brk expression in zone 1. However, it also possible that Mad acts more directly through the Mad-binding sites.