Models of ephrin-B1–driven cell segregation. Based on our work in the mouse embryo, we conclude that unidirectional forward signaling drives cell segregation. Possible mechanisms for unidirectional signaling in cell segregation include unidirectional regulation of repulsive migration and unidirectional regulation of cell adhesion. Based on our live imaging data, we propose a third mechanism, whereby unidirectional signaling leads to a differential in cortical actin between signaling and receiving cells. We suggest that this differential drives segregation by a mechanism that fits the differential interfacial tension hypothesis.