Alpha-crystallin domain found in the small heat shock protein (sHsp) alphaA-crystallin (HspB4, 20kDa). sHsps are molecular chaperones that suppress protein aggregation and protect against cell stress, and are generally active as large oligomers consisting of multiple subunits. Alpha crystallin, an abundant protein in the mammalian lens, is a large (700 kDa) heteropolymer composed of HspB4 and HspB5, generally in a molar ratio of HspB4:HspB5 of 3:1. Only trace amounts of HspB4 are found in tissues other than the lens. HspB5 does not belong to this group. Mutations inHspB4 have been associated with Autosomal Dominant Congenital Cataract (ADCC). The chaperone-like functions of HspB4 are considered important for maintaining lens transparency and preventing cataract.