(A) Expression of hoxPG4 genes in st21–26 embryos, shown in lateral view with anterior to the left. Neural expression is seen in the posterior hindbrain and/or spinal cord for each gene. To facilitate comparison of this neural expression across time and between genes, the pharyngeal arches (PA) that are adjacent to the anterior neural expression boundaries are labelled. Arrows indicate anterior spread of dorsal neuronal expression domains at st25–26. (B) A model of the duplication events that are inferred to have occurred in the lamprey/cyclostome lineage, leading to the 6 Hox clusters in Sea lamprey. Gene names in (A) are shaded to reflect these paralogy relationships. Scale bars: 200μm. e, endostyle; PA, pharyngeal arch; r, rhombomere.