etinoschisis 1 is an X-linked recessive eye disease leading to moderate to severe visual handicap at late adulthood. The RS1 gene, located on chromosome Xp22.13, encodes a 23 kD protein, retinoschisin, a secretory discoidin-domain protein expressed exclusively in retina. The RS1 gene is expressed only in the photoreceptor layer, but the protein product is present both in the photoreceptors and within the inner portions of the retina. RS1 functions as an octamer and is implicated in cell-cell interactions and cell adhesion. It plays an important, as yet unknown function during retinal development. The lesions in Retinoschisis 1 are thought to be related to a defect in retinal Mueller cells, the principal glial cells of the retina. Affected males usually have mild visual impairment during childhood, often discovered at routine visus controls at schools. Some patients may stay symptomless till adulthood. Most patients retain reasonable vision until fifth or sixth decades when some deterioration usually takes place. Total blindness is an exception. At funduscopy during early stages of the disease perifoveal radial microcysts are seen in the deep nerve fiber layer in a cartwheel-like pattern. Detachment of the retina and occasionally complete retinal atrophy with sclerosis of the choroid is seen during the later stages of the disease. Heterozygote female carriers are unaffected. Diagnosis of RS1 is made by funduscopy. Gene tests can be used if the mutation of the family or the founding mutation(s) of the population are known.