Transgenic embryos were generated carrying a Hoxb-8 transgene under control of the mouse RAR beta 2 promoter, which extends the normal expression domain to more anterior regions of the embryo. These embryos showed mirror-image duplications in the forelimb, analogous to the duplications observed in chick in response to transplantation of a ZPA to the anterior margin of the limb bud. Examination of Sonic hedgehog, Fgf-4, and Hoxd-11 gene expression confirmed that a second ZPA had been generated at the anterior side of the limb bud. Besides other alterations, posterior homeotic transformations of axial structures were observed, involving the first spinal (Froriep's) ganglion and several cervical vertebrae.