So calmodulin is a regulatory protein that is used to detect changes in calcium ion concentrations. If the cytoplasmic concentration of calcium ions raises to about 500 nm, so five times what it should be under steady state conditions, calmodulant will begin binding these calcium ions. And a single calmodulant, because it contains a few of these, can actually bind four individual calcium mines. So one here, one here, and two here, which is not shown here. Now, upon binding, the calcium induces a structural change in the calmodulent that activates it. So it exposes these hydrophobic regions of calmodulen which weren't exposed before.