Forward signaling is critical for cell segregation in Efnb1^Δ/+ neuroepithelium. E8.5 Efnb1^ΔXGFP/+ mouse embryos with various combinations of Eph receptor mutations. LacZ, forward signaling dead allele. Numbers of embryos analyzed are presented in Table S2. The GFP signal was adjusted in a nonlinear fashion to assist discrimination of the ephrin-B1–negative population. Bars, 100 µm.