γ-Crystallins are major structural components of the lens fiber cells in amphibians and mammals. Many dominant inherited cataracts in humans and mice have been shown to map within the γ-crystallin gene cluster. Several transcription factors, including PAX6 and SOX proteins, have been suggested as candidates for crystallin gene regulation. Here we show that the targeted deletion of Sox1 in mice causes microphthalmia and cataract. Mutant lens fiber cells fail to elongate, probably as a result of an almost complete absence ofγ-crystallins. It appears that the direct interaction of the SOX1 protein with a promoter element conserved in all γ-crystallin genes is responsible for their expression.