The term saturation refers to the fraction of total binding sites that are occupied at any given time. When more than one type of ligand can bind to a binding site, competition ensues.
An equilibrium exists between unbound ligands and bound ligands.
Binding sites are often an important component of the functional characterization of biomolecules. For example, the characterization the binding site of a substrate to an enzyme is essential to model the reaction mechanism responsible for the chemical change from substrate to product.
A more specific type of binding site is the transcription factor binding site, present on DNA.
See also 
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