Fig. 7. Model to show how transcription factor activity is integrated at the Xag1 promoter to bring about cement gland-specific expression. Markers of differentiation, such as Xag1, are activated at the end of gastrulation in the cement gland primordium (CG), which is defined by the overlap of up to three larger domains: anterodorsal (AD), ventrolateral (VL) and ectodermal outer layer (EO). Surface views of early neurula stage embryos are shown for the AD and VL panels, and sagittal section schematics for the EO and cement gland panels. The AD domain is defined by otx2 expression, the VL domain by BMP4 or some downstream readout, such as activated Smad1, while an unknown factor (XXX) defines the outer ectodermal layer. Xag1 expression in the cement gland primordium requires an ATF/CREB factor, which binds to the CRE and lies downstream of otx2. This acts in cooperation with an Ets factor that binds to the EBS and may lie downstream of BMP4 and/or XXX. Other inputs may influence Xag1 expression, but are not sufficient on their own to drive expression. CG, cement gland; ar, archenteron; Bc, blastocoel; A, anterior; P, posterior; D, dorsal; V, ventral.