Fig. 1. Mouse Hox genes and their early temporal and spatial colinearity of expression. (A) The four Hox clusters (a to d). Hox genes with the same number (1 to 13) are called paralogs. Three paralog groups are shown (2, 4 and 9) in colour to illustrate the early temporal co-linearity of their expression, as shown in B. (B) Mouse embryos at: embryonic day (E) 7.2, late streak (LS) stage; E7.5, neural plate (NP) stage; and E7.7, head fold (HF) stage. (C) An E10.5 embryo, showing the spatial co-linearity of Hox gene expression. Hox2 paralogs begin to be expressed earlier, and Hox4 and Hox9 paralogs progressively later, in the posterior part of the primitive streak (ps, indicated by a grey line on the posterior side of the embryos in B). At E10.5, the expression domains of the 3' genes extend to more anterior positions than that of the more 5' genes. For each gene, the expression boundary is more anterior in the nervous system than in the mesoderm. mes, mesoderm; nt, neural tube. Actual widths of embryos at widest point: LS, 0.26 mm; NP, 0.44 mm; HF, 0.60 mm; E10.5, 4.1 mm.