Crystallins are the major proteins of the vertebrate eye lens, where they maintain the transparency and refractive index of the lens. Crystallins are divided into a, b and g families, and the b- and g-crystallins also compose a superfamily. Crystallins usually contain seven distinct protein regions, including four homologous motifs, a connecting peptide, and N- and C-terminal extensions.a-crystallins consist of three gene products, aA-, aB- and aC-crystallin, which are members of the small heat shock protein family (HSP 20). a-crystallins act as molecular chaperones by holding denatured proteins in large soluble aggregates. However, unlike other molecular chaperones, a-crystallins do not renature these proteins. Expression of aA-crystallin is restricted to the lens and defects of this gene cause the development of autosomal dominant congenital cataracts (ADCC). The human aB-crystallin gene product is expressed in many tissues, including lens, heart and skeletal muscle. Elevated expression of aB-crystallin is associated with many neurological diseases, and a missense mutation in this gene has co-segregated in a family with a Desmin-related myopathy.