In the mouse, the embryonic ectoderm development (eed) region is defined by deletions encompassing the albino (c) locus of chromosome 7. The region is located 1-2 cM distal to the c locus and was of undetermined size. Embryos homozygous for deletions removing eed display defects in axial organization during gastrulation. Two loci, identified by chemical mutagenesis, are known to map within the eed interval. One, l7Rn5, probably represents the gene required for gastrulation. The second, l7Rn6, is required for survival after birth. fit1, a third locus identified by chemical mutagenesis, maps distal to the eed interval and is also required for survival after birth. A 900-kb YAC contig has been constructed, and deletion breakpoints defining the limits of the regions containing these loci have been localized. Their positions place the eed region within a maximum 150-kb interval at the proximal end of the contig, while fit1 maps to a 360-kb interval within the middle of the contig. Several clusters of rare-cutting restriction sites map within these regions and represent potential locations of candidate genes.