Over the past few years, several genes involved in the development of the tetrapod limbs have been isolated and characterized. Thus, it becomes possible to study some of the developmental mechanisms responsible for the morphogenesis of these complex structures through a novel approach. Alternatively, these genes can be used as tools to examine, at the molecular level, some aspects of the fascinating question of the fin to limb transition. Such an approach provides additional facts which illustrate both the similarities an differences in the morphologies of these homologous structures. In particular, the study of zebrafish (Danio rerio) Hox genes suggests a scenario for the fin-limb transition in which the distal parts of the autopods (the digits) are neomorphic structures produced by unequal proliferation of the posterior part of an ancestral appendix. Such an asymmetric morphogenesis could he the reason why tetra-pods have their major limb axes bent anteriorly, towards their distal ends, while the homologous fin axis extends straight from proximal to distal.