Figure 2.

Number of possible BFs in a node and the topologies of a network. (A) The number of possible BFs for a particular node depends on the number of inputs or regulators of the node. In each possible state of its inputs, the node can assume a 0 or 1 expression value. Thus, $2^{2^i}$ possible BFs are available to describe the regulation of a node with i inputs. (B) The number of possible topologies of a network depends on the number of nodes. In a network, each node can interact with itself and any other node. Thus, n² possible interactions exist. Because each interaction can either exist or not exist, $2^{n^2}$ possible topologies describe a network with N nodes. E, Exist, and D, Do not exist.