Retinitis pigmentosa (RP) is a group of progressive hereditary disorders of the retina in which various modes of inheritance have been described. Here, we report on X linked RP in nine families with constant and severe expression in carrier females. In our series, however, the phenotype was milder and delayed in carrier females compared to hemizygous males. This form of X linked RP could be regarded therefore as partially dominant. The disease gene maps to chromosome Xp2.1 in the genetic interval encompassing the RP3 locus (Zmax=13.71 at the DXS111O locus). Single strand conformation polymorphism and direct sequence analysis of the retinitis pigmentosa GTPase regulator (RPGR) gene, which accounts for RP3, failed to detect any mutation in our families. Future advances in the identification of X linked RP genes will hopefully help to elucidate the molecular basis ofthis X linked dominant RP.

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Juvenile retinoschisis (RS) is an X linked recessive vitreoretinal disorder for which the basic molecular defect is unknown. The gene for RS has been previously localised by linkage analysis to Xp22.1-p22.2 and the locus order Xpter-DXS16-(DXS43, DXS207)-RS-DXS274-DXS41-Xcen established. To improve the resolution of the genetic map in the RS region, we have isolated a highly polymorphic microsatellite at DXS207, which displays at least nine alleles with a heterozygosity of 0-83. Using this microsatellite and four other Xp22.1-p22.2 marker loci, DXS16, DXS43, DXS274, and DXS41, we performed pairwise and multilocus linkage analysis in 14 kindreds with RS. The microsatellite was also typed in the CEPH (Centre d'Etude du Polymorphisme Humain) reference families. Tight linkage was found between RS and DXS207 (Z(O) = 14-32 at 0 = 0O0), RS and DXS43 (Z(0)=8-10 at 0=00), and DXS207 and DXS43 (Z(0) = 4031 at 0 = 00). Our linkage results combined with data previously reported suggest that the DXS207-DXS43 cluster is located less than 2 cM telomeric to the RS locus. The microsatellite reported here will be a very useful marker for further linkage studies with retinoschisis as well as with other diseases in this region of the X chromosome.(J Med Genet 1993;30:300-3) Despite the variability in the clinical expression of the disease, no evidence for genetic heterogeneity for RS was found. These results have already rendered genetic prediction feasible in many RS families.2 A more precise mapping of the RS locus is a prerequisite for positional cloning. Accuracy of genetic mapping, including identification of recombinants which provide crucial information for orienting the gene search, was limited by the lack of highly informative markers in the region of the RS gene. We have identified a highly informative microsatellite marker at DXS207. Our linkage results suggest that this marker is located less than 2 cM telomeric to the RS locus. Patients and methods PATIENTSFourteen unrelated families with X linked recessive retinoschisis were analysed, seven of which have been used in a previous linkage study reported by Kaplan et al.2 The diagnosis was, in each case, confirmed by careful ophthalmological examination. DNA analysis was performed on 181 subjects including 39 affected males.