Editing Gene expression programming (section)

==Cells and code reuse==
In gene expression programming, [[gene expression programming#Homeotic genes and the cellular system|homeotic genes]] control the interactions of the different sub-ETs or modules of the main program. The expression of such genes results in different main programs or cells, that is, they determine which genes are expressed in each cell and how the sub-ETs of each cell interact with one another. In other words, homeotic genes determine which sub-ETs are called upon and how often in which main program or cell and what kind of connections they establish with one another.

===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.

===Multiple main programs and multicellular systems===
Multicellular systems are composed of more than one [[gene expression programming#Homeotic genes and the cellular system|homeotic gene]]. Each homeotic gene in this system puts together a different combination of sub-expression trees or ADFs, creating multiple cells or main programs.

For example, the program shown in the figure was created using a cellular system with two cells and three normal genes.

[[File:Expression of a multicellular GEP system with 3 ADFs and 2 main programs.png|thumb|Expression of a multicellular system with three ADFs and two main programs. a) The chromosome composed of three conventional genes and two homeotic genes (shown in bold). b) The ADFs encoded by each conventional gene. c) Two different main programs expressed in two different cells.]]

The applications of these multicellular systems are multiple and varied and, like the [[gene expression programming#Multigenic chromosomes|multigenic systems]], they can be used both in problems with just one output and in problems with multiple outputs.