We investigated the XLRS1 gene in Japanese patients with retinoschisis (RS). All exons of the XLRS1 gene were sequenced in 14 males, including a pair of monozygotic twins, from 11 individual families with RS and five of their mothers who are asymptomatic but diagnosed as carriers. Six kinds of missense mutations and a nonsense mutation, including six novel mutations, were detected in all 14 patients and carriers. Mutations in the XLRS1 gene are also responsible for RS in non-Caucasian patients. Most Japanese RS cases are caused by an XLRS1 gene defect. A novel mutation, Glu72Lys, was found in four families, suggesting a common mutation in the Japanese population. Clinical features of RS patients with both the Glu72Lys and Pro193Leu mutations indicate that a genotype-phenotype correlation is not recognized in RS.