In order to define the function of genes believed to be important in murine development, the technique of homologous recombination will be applied to mutate the Hox 2.1, Hox 2.6 and AFP genes in cultured ES cells. It should be possible to select or detect clones of cells which carry mutations caused by homologous recombination within these genes using either promoterless neo vectors and G418 selection, or MCLneo vectors and a tk or hprt based negative selection scheme. Selected clones carrying the appropriate genetic modification will be used to construct chimaeras so that these specific genetic changes can be introduced into the mouse germ line. This will facilitate a direct experimental test of the gene's function in development for the first time, and will eventually lead to the genetic complementation analysis of these loci. The success of homologous recombination applied to the stem-cell chimaera system will provide a general technique with which to develop animal models of human genetic disease.