The origin of different mesodermal tissues during gastrulation and the developmental lability of mesodermal precursors can be mapped by transplanting marked epiblast cells to the same or a different position in a host egg cylinder, and assessing the subsequent fate of transplanted tissue. This information provides the context for assessing the role of particular patterns of gene expression during mesoderm formation and differentiation. For example, the stability of Hox gene expression can be examined by transplanting transgenically marked somites that express a particular Hox gene to a position in the somite file where it is not normally expressed. Such experiments can reveal not only the cues required for Hox gene expression but also the relevance of a circumscribed pattern of Hox gene expression to a specific developmental fate. A different approach to resolving gene function is to mix mutant cells known to affect mesoderm formation with normal cells and to determine the cell autonomy of mutant cells in a normal environment. Homozygous Brachyury (T/T) embryonic stem cell lines have been isolated and injected into normal blastocysts. The presence of T/T cells in chimeras results in mesodermal defects similar to those seen in the intact mutant.