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Lenses from wild type mice and Cx46fs380 heterozygotes and homozygotes were studied at 1-3 months of age to determine refractive properties (by photographing the lenses above a 200-mesh electron microscopy grid), focusing ability (by laser scanning), and membrane potential, gap junctional coupling and hydrostatic pressure (by microelectrode studies of intact lenses). Lenses from homozygous mice significantly distorted the EM grid pattern and had reduced focal lengths compared to wild type lenses. The differentiating and mature fiber cells of heterozygous and homozygous Cx46fs380 animals had significantly depolarized membrane potentials; this alteration was more severe in homozygotes. Gap junctional coupling was decreased in both mature and differentiating fibers in heterozygous and homozygous Cx46fs380 mice; in homozygotes, the mature fibers had no detectable coupling. Hydrostatic pressure was also significantly increased in the lenses of mutant mice (homozygotes> heterozygotes). Homozygous Cx46fs380 mice have alterations of lens refractive and focusing properties that are detectable before the appearance of cataracts and are consistent with myopia. Changes in membrane potential, gap junctional conductance, and hydrostatic pressure are present in all mutant lenses and precede cataract formation (at least in heterozygotes). Taken together, these data suggest that expression of Cx46fs380 causes a series of events affecting lens electrophysiological parameters, water flow, and optical properties prior to development of visible opacities.