The homoebox-containing genes of the Hox-5 complex are expressed in different but overlapping domains in limbs during murine development. The more 5' the position of these genes in the complex, the later and more distal is their expression. Antero-posterior differences are also observed. A model is proposed that accounts for the establishment of these expression domains in relation to the existence of a morphogen released by the zone of polarizing activity. Comparison of these observations with the expression patterns of the genes of Hox complexes in the early embryo suggests that similar molecular mechanisms are involved in the positional signalling along the axes of both the embryonic trunk and the fetal limbs.