Occupancy

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In medicine and pharmacology occupancy is the amount amount, or number, of receptors which are 'occupied' by an agonist.

For a reaction to occur at all a receptor must be 'occupied'.

But it is unusual to need 100% occupancy to observe 100% response from a tissue (if this is the case it is unlikely that a 100% response will ever be observed). Often it is a matter of less than 10%.