Developmental genetics in Drosophila led to the isolation of the homeotic genes which are involved in the cellular positional information. In vertebrates, homologous genes have been characterized and play similar roles in the spatial determination. However, the subtle mechanisms by which positional identity is specified by the Hox genes co-expression are little known. The Hoxc-8 null mutation can induce homeosis at the anterior margin of Hoxc-8 expression, where the mosaicism observed suggests that, at least in the segmented paraxial mesoderm, the number of cells expressing a Hox gene is a determinant parameter. A combinatorial model is also suggested by several genetic experiments where the level of Hox genes expression is modified. Expression of some related Hox genes in Hoxc-8 homozygous mutants is not altered, suggesting the absence of genetic interregulations between these genes.