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 Alpha, Beta, and Gamma families, and the Beta- and Gamma-crystallins also comprise a superfamily. Crystallins usually contain seven distinctive protein regions, including four homologous motifs, a connecting peptide, and N- and C-terminal extensions. Beta-crystallins constitute the major lens structural proteins, and they associate into dimers, tetramers, and higher order aggregates. The Beta-crystallin subfamily is composed of several gene products, including Beta A1-, Beta A2-, Beta A3-, Beta A4-, Beta B1-, Beta B2- and Beta B3-crystallin. The Beta A1- and Beta A3-crystallin proteins are encoded by a single mRNA. They differ by only 17 amino acids, and Beta A1-crystallin is generated by use of an alternate translation initiation site.