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Fig. 2. The purpose of the BMP gradient peak is to set up the zen domain. All views of embryos are dorsal, except K (lateral). Embryos are of the genotypes wild type 2X dpp (A-C), 4X dpp (D-F), 4X dpp, zen^w36 (G-I) and UAS-zen driven by maternal-Gal4 (J-L; see Materials and methods), and were stained with anti-phospho Mad (PMad) antibodies (A,D,G,J), or hybridized with zen (B,E,H,K) or Race (C,F,I,L) RNA probes. Increasing the dose of dpp to four copies broadens the domain of peak level PMad to 10-12 cells (D; see inset for higher magnification view of one or two rows of cells, turned sideways). The domain of zen (E) and Race (F) broaden likewise. In zen mutant embryos with 4X dpp, the zen domain broadens (H), but that of Race does not (I), presumably because Zen proteins are absent (Rushlow et al., 1987). When zen is ubiquitously expressed throughout the embryo (K), the Race domain broadens to encompass the lower-level PMad domain (L), indicating that lower levels of PMad can activate Race if Zen is present. Ectopic Race expression is also observed in the posterior region (L, arrow) where PMad is present.

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