Hox gene
Chicken development is a wonderful example of this specificity of Hox gene expression in regard to limb development. The most 3’ Hoxc genes (HOXC4, HOXC5) are expressed only in the anterior limbs in chickens, while the more 5’ genes (HOXC9, HOXC10, HOXC11) are expressed only in the posterior limbs. The intermediate genes (HOXC6, HOXC8) are expressed in both the upper and lower limbs in chickens.