Abstract: When calcium ions enter the cytosol, it is a stimulatory signal for cellular events. The calcium sensor calmodulin picks up the change in calcium concentration and relays this information to its more than 300 downstream interaction partners. In this way, calmodulin affects cellular processes such as fertilization, muscle contraction, neuronal firing, and apoptosis. That is, calmodulin is involved in (nearly) everything! The significance of calmodulin is emphasized by the fact that we all carry three different genes (CALM1,2,3) on different chromosomes that encode the exact same calmodulin protein, and these are all expressed in all cell types. Moreover, throughout vertebrate evolution, the protein sequence has remained completely unchanged.