Master control gener der inducerer en kaskade af gene ekspression, der fører til udviklingen af REGIONALE STRUKTURER
Hox genes (from an abbreviation of homeobox) are a group of related genes that control the body plan of an embryo along the anterior-posterior (head-tail) axis. After the embryonic segments have formed, the Hox proteins determine the type of segment structures (e.g. legs, antennae, and wings in fruit flies or the different types of vertebrae in humans) that will form on a given segment. Hox proteins thus confer segmental identity, but do not form the actual segments themselves 
Hox genes are defined as having the following properties:
- their protein product is a transcription factor
- they contain a DNA sequence known as the homeobox
- in many animals, the organization of the Hox genes on the chromosome is the same as the order of their expression along the anterior-posterior axis of the developing animal, and are thus said to display colinearity.