Homeobox gene XlHbox 1 is expressed in a mesodermal gradient in vertebrate forelimbs with maximal expression anteriorly and proximally and may encode positional values. In chick wing buds, anterior cells can be reprogrammed to form posterior structures by grafts of polarizing region tissue and by beads soaked in retinoic acid (RA), which is a good candidate for an endogenous morphogen. Applications of RA anteriorly or at the bud apex, treatments which produce duplicated digits or truncations respectively, substantially increase the extent of mesodermal XlHbox 1 expression. Polarizing region grafts that also produce additional digits lead to a moderate increase. The effects of RA application and the behaviour of transplanted tissue show that only anterior cells are competent to express XlHbox 1 and that expression is cell autonomous. Ectodermal expression in wing buds is enhanced by RA but not by polarizing region grafts and ectoderm/mesoderm recombinations show that the mesoderm is irreversibly affected. The changes in mesodermal expression do not fit the predictions of the simple model that XlHbox 1 encodes anterior positional values but are correlated with a series of novel malformations of the shoulder girdle which, in normal wing buds, is derived from cells expressing XlHbox 1.