Fig. 8. Models of Hox response to FGF based on different effects of FGF on Cdx expression and Hox accessibility. (A) In normal development, Hox loci (colored boxes) are progressively opened over time in an anterior to posterior direction. During this period, Cdx expression domains (grey shaded areas) are gradually regressing toward the caudal end of the neural tube. As each Hox gene in a complex is believed to be accessible at a slightly different time, when it becomes accessible it is exposed to a different pattern or level of Cdx expression, to which it can respond. Hence the final boundary of a given Hox gene is determined by two parameters: the time when the Hox locus is accessible and the position of the Cdx anterior boundary at this particular time. As the embryo develops, Hox loci become accessible in a domain where Cdx is not expressed so they are not capable of being induced. This sets up the nested patterns of expression shown at the right. (B) The addition of FGF leads to an anterior expansion and maintenance of Cdx domains of expression over time and leads to an extended accessibility of Hox complexes along the entire AP axis. This dual effect induces an anteriorization of Hox domains in the neural tissue.