The segmental migration of neural crest cells is disrupted in explants treated with EphA4- or ephrinA-Fc. Trunk explants containing premigratory neural crest cells were isolated, as previously described (17). Explants were treated with 10 μg/ml of EphA4-Fc (A), EphA7-Fc (B), ephrin-A2-Fc (C), ephrin-A5-Fc (D), or Fc (E, controls). The disposition of neural crest cells was then examined using HNK-1 antibody (red) and ephrin-B1 antibody (green) was used as a marker for the caudal half somite. Brackets mark the extent of a single somite: r, rostral; c, caudal. Neural crest cells invade the caudal half somites in EphA4-Fc-, ephrin-A2-Fc-, and ephrin-A5-Fc-treated explants (A, C, D: arrows). In EphA7-Fc and Fc controls, neural crest cells migrate in the rostral half somite, typical of normal embryos (B, E).