Editing Gene expression programming (section)

===Homeotic genes and the cellular system===
Homeotic genes have exactly the same kind of structural organization as normal genes and they are built using an identical process. They also contain a head domain and a tail domain, with the difference that the heads contain now linking functions and a special kind of terminals – genic terminals – that represent the normal genes. The expression of the normal genes results as usual in different sub-ETs, which in the cellular system are called ADFs (automatically defined functions). As for the tails, they contain only genic terminals, that is, derived features generated on the fly by the algorithm.

For example, the chromosome in the figure has three normal genes and one homeotic gene and encodes a main program that invokes three different functions a total of four times, linking them in a particular way.

[[File:Expression of a unicellular GEP system with three ADFs.png|thumb|Expression of a unicellular system with three ADFs. a) The chromosome composed of three conventional genes and one homeotic gene (shown in bold). b) The ADFs encoded by each conventional gene. c) The main program or cell.]]

From this example it is clear that the cellular system not only allows the unconstrained evolution of linking functions but also code reuse. And it shouldn't be hard to implement [[Recursion (computer science)|recursion]] in this system.