receptor sites
There are many phrases that express this idea. See the link.
"Receptor sites are like baseball mits waiting to catch a baseball - in this case oestrogen. The action of genistein is to 'fool' the receptor sites, making them ..."
"Receptor sites are like little antennae on the cell that receive specific cytokines like a lock fits into a key"
There are different types of parking spaces: regular, handicapped, motorcycles and maybe large ones for trucks and RVs. Receptor sites come in many different flavors that are specific for their particular ligand.
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