In all eukaryotic cells, and particularly in neurons, Ca2+ ions are important second messengers in a variety of cellular signaling pathways. In the retina, Ca2+ modulation plays a crucial function in the development of the visual system's neuronal connectivity and a regulatory role in the conversion of the light signal received by photoreceptors into an electrical signal transmitted to the brain. Therefore, the study of retinal Ca2+-binding proteins, which frequently mediate Ca2+ signaling, has given rise to the important discovery of two subfamilies of these proteins, neuronal Ca2+-binding proteins (NCBPs) and calcium-binding proteins (CaBPs), that display similarities to calmodulin (CaM). These and other Ca2+-binding proteins are integral components of cellular events controlled by Ca2+. Some members of these subfamilies also play a vital role in signal transduction outside of the retina. The expansion of the CaM-like protein family reveals diversification among Ca2+-binding proteins that evolved on the basis of the classic molecule, CaM. A large number of NCBP and CaBP subfamily members would benefit from their potentially specialized role in Ca2+-dependent cellular processes. Pinpointing the role of these proteins will be a challenging task for further research. (C) 2002 Elsevier Science.