Position-specific expression of homeoprotein gradients in different feather tracts.

Homeoproteins have been shown to be expressed in defined bands along the anteriorposterior (A-P) body axis and are involved in determining the patterns of insect appendages.' In vertebrates, homeoproteins are also expressed in anterior-posterior (A-P) gradient in the spinal cord. Recently, homeoproteins were also shown to be distributed in a gradient in developing limb buds2.3 and feather buds: thus suggesting that homeoproteins may be involved in setting up the A-P axes of cell fields at different levels, first for the body axis, then for the limb axis, and finally for the feather a x k 4 Here, we further examine the orientation of the homeoprotein gradient at the level of the feather tract using the procedure of Chuong et aL4 We found that there are different but definite gradient axes in different tracts. Thus there is a position-specific homeoprotein expression pattern in each feather bud. Together with previous findings: the observations are summarized as follows.