Expression of Hox genes in 5-d fugu embryos. (a) Hoxa2a is expressed in an apparently novel striped pattern in r1 and r2. (b) Hoxa2b is expressed in r2-r5 as in other vertebrates. (c) Hoxa3a has an anterior boundary at the r4/r5 border as in other vertebrates. (d) Hoxb3a has strong expression in r5 and r6 and weak expression in r4 as in zebrafish, but with weak expression extending into r3 in fugu and not in zebrafish (Prince etal. 1998b). (e) Hoxd3a expression mimics that of zebrafish, with an anterior limit at the r5/r6 border, and a small lateral/ventral group of r5 cells (Prince etal. 1998b). Expression of eng1b atthe midbrain/hindbrain border serves as a marker for rhombomere position in f-i. (f) Hoxa4a is expressed with a diffuse anterior expression boundary near the r7/r8 border as in zebrafish (Prince et al. 1998b). (g) Hoxc4a is expressed with a diffuse anterior boundary, and as in zebrafish, the border lies within r7, atleastmedially, and interneurons express eng1b as in zebrafish (Force et al. 1999). (h) Hoxd4a transcript tissue distribution as in other vertebrates. (i) Hoxd4b has a more caudal anterior expression border than Hoxd4 in other vertebrates. (j) Hoxd4a in lateral view. (k) Hoxd4b in lateral view. (l) Hoxd4a in dorsal view showing fin buds. (m) Hoxd4b in dorsal view showing fin buds. Scale bar, 100 μ. (cc) Cranial crest; (f) fin buds.