Calcium concentration microdomains (CCMs) are sites in a cell's cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 with a localised high calcium ion (Ca²⁺) concentration. They are found immediately around the intracellular opening of calcium channel

Calcium channel

A Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...

s; when a calcium channel opens, the Ca²⁺ concentration in the adjacent CCM increases up to several hundred millimolar (mM). These microdomains take part in calcium signaling

Calcium signaling

Calcium is a common signaling mechanism, as once it enters the cytoplasm it exerts allosteric regulatory effects on many enzymes and proteins...

, which has a diverse range of potential outcomes.

Calcium concentration microdomains can be visualised with fluorescence microscopy by using aequorin

Aequorin

Aequorin is a photoprotein isolated from luminescent jellyfish and a variety of other marine organisms...

as a reporter protein.