Fig. 1. The threshold response of C15 to the Mad-P gradient depends on both Smads and Zen. Dorsal views of late stage 5/early stage 6 embryos with anterior to the left. The widest area of expression is delineated with a white line labelled to show the width in number of cells. (A) Wild-type embryo hybridized with C15 probes. Transcripts are present in a five- to ten-cell-wide dorsal strip of cells in the main body region and a faint spot in the presumptive head region. Cells in this region are magnified in the inset, which lies perpendicular to the embryo. (B) Wild-type embryo stained with an antiphospho-Smad1 antibody, which also recognizes Mad-P. Strong staining is observed in a five- to six-cell-wide stripe with less staining in three to four cells to either side of the stripe (see magnified inset). In terminal regions, the Mad-P stripe can be up to 16 cells wide. (C) Wild-type embryo hybridized with Race probes. Transcripts are present in a four- to six-cell-wide stripe. (D) Wild-type embryo hybridized with tup probes. The dorsal stripe is ten to 14 cells wide. (E-H) Mutant embryos hybridized with C15 probes. C15 is absent in dpp^hr4 homozygotes (E) and zen^w36 homozygotes (F). C15 is in a broad domain in sog^YS06 hemizygous embryos (G). The C15 pattern broadens to about 26 cells at its widest in a maternal GAL4>UAS-zen embryo (H).