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Fig. 4 hoxb1 (purple) and krox-20 (red) expression in wild-type (A) and val- (B,C,D) embryos, rhombomeres are numbered. At the 5s stage, there are no obvious differences in the hoxb1 expression pattern between normal (A) and mutant (B) embryos. At the 10s stage the posterior limit of hoxb1 expression in r4 is sharp in wild-type embryos (see Fig. 2Aiii), but not in mutant embryos (C). By the 20s stage there is diffuse hoxb1 staining throughout rX (D). hoxb1 and hoxb4 probes were combined to show expression of both of these genes in wild-type (E,G,I) and val- (F,H,J) embryos. At the 10s stage (E) and the 20s stage (G – dorsal view; I – lateral view) there is a clear zone of nonexpression overlying r5 and r6 in wild-type embryos. In val- embryos at 10s (F) the size of this domain is reduced and some expressing cells are visible within the domain. By the 20s stage (H – dorsal view; J– lateral view) there is expression throughout most of rX, although expression is reduced in dorsal rX in the region where krox-20 expression is expected to localize (indicated by bracket in J). Scale bar, 50 μm.