Control coefficients measure the response of a biochemical pathway to changes in enzyme activity. The response coefficient, as originally defined by Kacser and Burns, is a measure of how external factors such as inhibitors, pharmaceutical drugs, or boundary species affect the steady-state fluxes and species concentrations. The flux response coefficient is defined by:
where  is the steady-state pathway flux. Similarly, the concentration response coefficient is defined by the expression:
where in both cases  is the concentration of the external factor. The response coefficient measures how sensitive a pathway is to changes in external factors other than enzyme activities. 
The flux response coefficient is related to control coefficients and elasticities through the following relationship:
Likewise, the concentration response coefficient is related by the following expression:
The summation in both cases accounts for cases where a given external factor, , can act at multiple sites. For example, a given drug might act on multiple protein sites. The overall response is the sum of the individual responses. 
These results show that the action of an external factor, such as a drug, has two components:
# The elasticity indicates how potent the drug is at affecting the activity of the target site itself. 
# The control coefficient indicates how any perturbation at the target site will propagate to the rest of the system and thereby affect the phenotype.
When designing drugs for therapeutic action, both aspects must therefore be considered.