X-linked congenital stationary night blindness (CSNB) is a nonprogressive retinal disorder characterized by impaired night vision, variably involving high myopia, nystagmus, decreased visual acuity, and strabismus. Linkage studies have identified two distinct loci for X-linked CSNB1 and CSNB2 on the short arm of chromosome X. The gene mutated in families displaying the "incomplete phenotype" of CSNB (i.e., CSNB2) has recently been identified. To identify novel candidate genes for the "complete form" of CSNB (i.e., CSNB1) we screened the physically vast region Xp11.3-Xp11.4 for cDNA sequences. This led us to identify and map the G protein coupled receptor (GPCR) gene GPR34 to Xp11.4 within 650 kb of the marker DXS993. Deletion screening via Southern blotting and direct sequencing of GPR34 revealed no mutations in 19 unrelated men with CSNB1, excluding a causal role in the disease. However, because of its expression in retinal and neural tissue and the involvement of GPCRs in transmembrane signal transduction, GPR34 remains a putative candidate gene for a number of ocular diseases which also map to the Xp11.4 region.