AbstractDuring vertebrate lens development, the anterior, ectoderm‐derived lens vesicle cells differentiate into a monolayer of epithelial cells that retain proliferative potential. Subsequently, they exit the cell cycle and give rise to posterior lens fiber cells that form the lens body. In the present study, we demonstrate that the transcription factor GATA‐3 is expressed in the posterior lens fiber cells during embryogenesis, and that GATA‐3 deficiency impairs lens development. Interestingly, expression of E‐cadherin, a premature lens vesicle marker, is abnormally prolonged in the posterior region of Gata3 homozygous mutant lenses. Furthermore, expression of γ‐crystallin, a differentiation marker for fiber cells, is reduced. This suppressed differentiation is accompanied by an abnormal cellular proliferation, as well as with diminished levels of the cell‐cycle inhibitors Cdkn1b/p27 and Cdkn1c/p57 and increased Ccnd2/cyclin D2 abundance. Thus, these observations suggest that GATA‐3 is essential for lens cells differentiation and proper cell cycle control. Developmental Dynamics 238:2280–2291, 2009. © 2009 Wiley‐Liss, Inc.