In Canada, Europe, and Israel, 2007 unrelated patients with isolated or autosomal recessive RP (80 from Ghent, 96 from Jerusalem, 245 from Madrid, 361 from Montpellier, 343 from Montreal, 143 from Naples, 215 from Nijmegen, and 524 from Tübingen) and 1824 ethnically matched individuals serving as controls (103 from Ghent, 100 from Jerusalem, 320 from Madrid, 56 from Montpellier, 361 from Montreal, 140 from Naples, 233 from Nijmegen, and 511 from Tübingen) were included in the study. The diagnosis of RP was based on ophthalmologic examination that included measurement of best-corrected visual acuity, Goldmann visual fields, slitlamp biomicroscopy, dilated indirect ophthalmoscopy, and fundus photography. In addition, full-field flash electroretinography (ERG), according to the guidelines of the International Society of Clinical Electrophysiology of Vision,41 was performed when Goldmann visual fields were measurable and indicated a preserved visual field with peripheral limits beyond the pericentral 10°. A broad definition of RP was used to avoid excluding patients with atypical features: progressive visual loss, night blindness, and abnormal ERG findings in a rod-cone pattern when recordable. Blood samples were obtained after informed consent forms were signed. Ethics approval was given to all participating institutions, and the study conformed to the tenets of the Declaration of Helsinki. Participants were evaluated again after the identification of the BBS1 gene defects, since they had previously received a diagnosis of nonsyndromic RP. Special attention was given to identifying systemic features associated with BBS,2 which include the primary features of RP, polydactyly, obesity, learning disabilities, hypogonadism in males, and renal abnormalities, and the secondary features of speech disorders, cataract, brachydactyly or syndactyly, developmental delay, polydipsia/polyuria (diabetes insipidus), ataxia, mild spasticity, diabetes mellitus, dental crowding or hypodontia, congenital heart diseases, and hepatic fibrosis. Where and when appropriate, patients were clinically reexamined or their records were again reviewed. Clinical evaluation included best-corrected visual acuity and slitlamp biomicroscopy of the anterior segment and retina. Additional examinations included kinetic and/or static perimetry and ERG according to the protocol of the International Society for Clinical Electrophysiology of Vision.