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The cell can be thought of both as an analogue and digital parallel computer. It is able to simultaneously decode analogue signals from its environment and make yes-or-no decisions in the context of present and previous experience. If enzymatic reactions and gene expression, for example, were constitutive processes, organisms would be unable to respond to stimuli and would be very inefficient. Therefore nearly all proteins are regulated, either at the level of expression, degradation or by interference with function. The direct modulation of protein function can be achieved in a number of ways, but can be grouped into two main categories: steric effects, and conformational change.