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 comprise a superfamily. Crystallins usually contain seven distinctive protein regions, including four homologous motifs, a connecting peptide, and N- and C-terminal extensions. b-crystallins constitute the major lens structural proteins. They associate into dimers, tetramers, and higher order aggregates. The b-crystallin subfamily is composed of several gene products, including bA1-, bA2-, bA3-, bA4-, bB1-, bB2- and bB3-crystallin. The bA1- and bA3-crystallin proteins are encoded by a single mRNA. They differ by only 17 amino acids, and bA1-crystallin is generated by use of an alternate translation initiation site. The genes for bA4-, bB1-, bB2- and bB3-crystallin are clustered on human chromosome 22q11, while the genes for bA3/A1- and bA2-crystallin map to human chromosomes 17q11 and 2q34, respectively.