The Hox genes are “physically linked in four chromosomal clusters (Hoxa, Hoxb, Hoxc, Hoxd), and their physical position on the chromosome seems to correlate with the time and place of expression. For example, the most 3’ HOXC genes (HOXC4, HOXC5) are expressed only in the anterior limbs (wings) in chickens, while the more 5’ genes (HOXC9, HOXC10, HOXC11) are expressed only in the posterior limbs (legs). The intermediate genes (HOXC6, HOXC8) are expressed in both the upper and lower limbs. Within the limb bud, expression also varies as a function of the position along the anterior-posterior axis. Such is the case with HOXB9, which is most highly expressed next to the AER, and decreases when moving anteriorly to posteriorly, resulting in the least HOXB9 expression next to the posterior ZPA. HOXB9 expression is inversely proportional to the level of Shh expression, which makes sense, as the ZPA secretes Shh.