(A) Quantification of the proximal to distal length of Rreb1 wild-type (Rreb1+/+) and heterozygous (Rreb1+/-) versus mutant (Rreb1-/-) littermates at E6.5 (3 litters) and 7.5 (5 litters). Each point represents an individual embryo. Total number of embryos is shown on the graph. Data is shown relative to the average wild-type/heterozygote proximo-distal length of each litter. Bars represent mean and IQR. ** p=≤0.005, unpaired t-test. (B) Brightfield images of wild-type (Rreb1+/+) and mutant (Rreb1-/-) littermates at embryonic day (E) 7.75, 8.0 and 9.0. Rreb1-/- embryos are smaller than wild-type littermates and do not show stage-appropriate morphological landmarks. (C) Quantification of relative number of somite pairs in E8.5–9.5 Rreb1 wild-type (Rreb1+/+) and heterozygous (Rreb1+/-) versus mutant (Rreb1-/-) littermates. Each point represents an individual embryo. Data is shown relative to the average somite number of each litter. Separate litters are indicated by different colored points. Bars represent mean and IQR. (E–F) Transverse cryosections of E9.0 Rreb1 heterozygous and homozygous mutant, Afp-GFP littermates. Boxes mark the regions shown in higher magnification in H. Asterisks mark the open neural tube and gut tube in Rreb1-/-. Sb, 50 μm. (G) Confocal optical sections of transverse cryosections from E9.0 embryos in the region of the notochord. From left to right, images show sections from rostral to caudal regions of the anterior embryo. Sb, 20 μm. Pr, proximal; Ds, distal; A, anterior; P, posterior; L, left lateral; R, right; D, dorsal; V, ventral; Am, amnion; Al, allantois; HF, headfolds; ML, midline; n, notochord; nt, neural tube; fg, foregut; ys, yolk sac; pcp, prechordal plate; hb, hindbrain; op, otic pit; ba, branchial arch; fb, forebrain; mb, midbrain.