Fig. 7. Comparison between the mouse and zebrafish Hox gene clusters. Only the numbers of genes and their paralogy groups are shown for comparison, along with the sizes (scale bar on top) of the various clusters. The four murine (M) clusters are shown with Hox genes in black and, below, their zebrafish (ZF) Hox gene counterparts in gray. Mouse Hoxa, Hoxb and Hoxc clusters have two zebrafish counterparts, whereas the mouse Hoxd cluster has a single orthologous cluster in zebrafish. On the right, both the number of genes per cluster and the sizes (kb) of the respective clusters are shown. Totals are given at the bottom, which show that despite zebrafish having the higher number of clusters and a slight increase in Hox gene number, the overall lengths obtained when all clusters are artificially considered together are virtually identical in the two animals. ^*, The position of the mouse Hoxb13 gene is not shown at the correct scale, as it is located 100 kb from Hoxb9 (see text). The Hoxb cluster is 200 kb, which makes the overall length of the four murine clusters even larger than that of the seven fish counterparts. , A ZFHoxDb `cluster' is shown, even in the absence of Hox genes, as this cluster was selectively lost, as indicated by the structure of the remaining genomic locus (Woltering and Durston, 2006).