While talking of signalling networks rather than programs, Brett Calcott has shown that positional information in the developing fruitfly embryo depends on this kind of Boolean structure, with limb bud development depending on the cells that differentiate into limb buds integrating one positive with two negative signals, so the buds develop in a regular pattern on the anterior midline of the embryo. Calcott shows that thinking of development in terms of these signalling networks with their Boolean structures has real explanatory value, for it enables us to explain how positional information, for example, can easily be reused in evolution. Wing spots on fruitflies can evolve very easily, for the networks that tell cells where they are on the wing already exist, so the evolution of the wingspot just need a simple mutational change that links that positional information to pigment production (Calcott 2014). Ron Planer agrees that gene regulation has this Boolean structure, and that we can, in effect, represent each gene as instantiating a conditional instruction. The “if” part of the conditional specifies the molecular conditions which turn the gene on; the “then” part of the conditional specifies the amino acid sequence made from the gene. As with Calcott, Planer goes on to point out that these conditional instructions can be and often are, linked together to build complex networks of control. Even so, Planer argues that while these are signalling networks, they should not be thought of as computer programs. For example, the combinations of instructions have no intrinsic order; we can represent each of the genes as a specific conditional instruction, but there is nothing in the set of instructions itself that tells us where to begin and end (Planer 2014).