Hox gene
The Hox genes, which define features along the anterior-posterior axis of a developing organism, determine at which points along the axis that limb buds will form. Though limbs emerge at different locations in different species, their positions always correlate with the level of Hox gene expression along the anterior-posterior axis. All limb buds must also rely on other signaling factors to obtain their forelimb or hindlimb identity; Hox gene expression influences expression of T-box proteins that, in turn, determine limb identity for certain organisms.