PURPOSE. To characterize the optical properties of lenses from mice deficient in the gene for lens intrinsic membrane protein-2 (Lim2), which encodes the second most abundant integral protein (Lim2) of lens fiber cell plasma membranes. METHODS. Lim2-deficient mice were derived from a library of gene-trap embryo stem cells. Genotyping was performed by polymerase chain reaction (PCR) amplification of tail genomic DNA and resequencing. Lim2 expression was analyzed by reverse transcription (RT)-PCR and Northern blotting of lens total RNA, immunoblotting of lens membrane extracts, and immunofluorescence confocal microscopy of lens sections. Lens morphology was assessed by light microscopy, and lens refractive properties were quantified with a laser imaging system. RESULTS. Genomic PCR amplification and resequencing indicated that the gene-trap vector had disrupted intron 3 of Lim2, effectively resulting in a null allele (Lim2Gt), as verified by RT-PCR amplification and sequencing, RNA blotting, immunoblotting, and immunofluorescence confocal microscopy. Heterozygous Lim2 gene-trap lenses (Lim2Gt/+) were morphologically indistinguishable from wild type, whereas homozygous Lim2 gene-trap lenses (Lim2Gt/Gt) consistently developed faint, central pulverulent cataracts. Laser imaging analysis indicated that rays passing through the peripheral cortex of the Lim2Gt/Gt lens were more strongly refracted than normal, suggesting that the internal gradient refractive index of the lens was disturbed. CONCLUSIONS. These data show that heterozygous loss of Lim2 is insufficient to trigger cataracts in mice, and they provide the first direct evidence that Lim2 plays a critical role in establishing the correct internal refractive properties of the crystalline lens.