The use of more than one homeotic gene results obviously in a multicellular system, in which each homeotic gene puts together a different consortium of genes.

Consider, for instance, the following chromosome:

It codes for three conventional genes and two homeotic genes (shown in blue). And its expression results in two different cells or programs, each expressing different genes in different ways (Figure 9). As you can see in Figure 9, ADF₁ is used twice in both cells; ADF₂ is used just once in both cells; and ADF₀ is only used in Cell₁.

Figure 9. Expression of a multicellular system with three Automatically Defined Functions. a) The chromosome composed of three conventional genes and two homeotic genes (shown in bold). b) The ADFs codified by each conventional gene. c) Two different main programs expressed in two different cells. Note how different cells put together different combinations of ADFs.

The applications of these multicellular systems are multiple and varied and, like the multigenic systems, they can be used both in problems with just one output and in problems with multiple outputs. In the former case, the best program or cell accounts for the fitness of the individual; in the latter, each cell is responsible for a particular facet in a multiple output task such as a classification task with multiple classes.

It is worth pointing out that the implementation of multiple main programs in Genetic Programming is virtually unthinkable and so far no one has attempted it.