The homoeo box is a 180-base pair (bp) DNA sequence conserved in Drosophila homoeotic genes, which regulate early development. These DNA sequences are present in open reading frames and have been identified in specific gene transcripts in Drosophila and Xenopus embryos; they possess structural features in common with genes encoding some DNA-binding proteins. Homologous homoeo box sequences have been detected in species ranging from insects and annelids to vertebrates. The high degree of sequence conservation (70-90%) among different species suggests a strong evolutionary relationship and implies a common role in embryonic development. To test this hypothesis, one approach we have used is to examine the patterns of genetic organization of homoeo box sequences in mouse and human for any similarities; the second approach is to localize the chromosomal map positions of homoeo box sequences in the two species. A similar genomic organization and chromosomal distribution of homoeo box sequences would argue for a conserved function and might shed light on their mechanism of action. Here, we describe experiments which show that two homoeo box loci map, respectively, to evolutionarily related regions on mouse chromosome 11 and human chromosome 17.