Fig. 8. An interactive hierarchy of zinc-finger transcription factors establishes trunk and gnathal/head segment types. In the trunk segments, TSH represses disco expression and directs segments along the trunk developmental pathway. SALM defines the boundary between the head and trunk (broken line) by repressing tsh in the gnathal segments. disco is expressed in the gnathal segments activating the gnathal development pathway. In this manner, the distribution of TSH and DISCO regionalizes the embryo. When combined with the HOM-C proteins, specific segment identities arise. Note that the HOM-C protein SCR is expressed in both the gnathal and trunk domains and yields a different identity depending upon which co-factor is present. DFD can establish either a maxillary or mandibular identity depending upon the presence of the Cap-n-collar (CNC) protein (Mohler et al., 1995). mn, mandibular; mx, maxillary; lb, labial; t2, second thoracic.