The connection however could be implemented in countless different ways since any first messenger can be coupled with any second messenger, which makes it imperative to use a code in order to guarantee biological specificity. In signal transduction, in short, we find the three characteristics of the codes:
(1) A correspondence between two independent worlds.
(2) A system of adaptors which give meanings to molecular structures.
(3) A collective set of rules which guarantee biological specificity. The effects that external signals have on cells, in conclusion, do not depend on the energy and the information that they carry, but only on the meanings that cells give them with rules that can be called signal transduction codes.

Contextual information

We have seen that there are only four types of second messengers, and yet the signals that they set in motion do have specific effects, i.e. they are able to find individual genes among tens of thousands. How this is achieved is still a mystery, but some progress has been made and so far the most illuminating discovery in the field has been the demonstration that signalling molecules have in general more than one function.
Epidermal growth factor, for example, stimulates fibroblasts and keratinocytes to proliferate, but has an anti-proliferative effect on hair follicle cells, whereas in the intestine it is a suppressor of gastric acid secretion. Other findings have proved that all growth factors can have three different functions, with proliferative, anti-proliferative, and proliferation-independent effects. They are, in short, multifunctional molecules (Sporn and Roberts, 1988).
In addition to growth factors, it has been shown that countless other molecules have multiple functions. Cholecystokinin, for example, is a peptide that acts as a hormone in the intestine, where it increases the bile flow during digestion, whereas in the nervous system it behaves as a neurotransmitter. Enkefalins are sedatives in the brain, but n the digestive system are hormones which control the mechanical movements of food.