Figure 3

Petri net representation of a Boolean entity. In this example gene 2 inactivates gene 1. Each of them is represented by two places in the Petri net. The upper (lower) part of the figure shows the Petri net before (after) the transition fires. When gene 2 is active, it inactivates gene 1. Therefore, the transition consumes a token from the active place of gene 1, and produces a token to its inactive place. The transition also consumes a token from the active place of gene 2, and produces a token to the same place. The latter consumption and production express the fact that gene 2 needs to be active in order to inactivate gene 1, but the inactivation itself does not change the level of gene 2.