Inherited retinal diseases such as autosomal dominant retinitis pigmentosa (adRP) are strikingly complex, with mutations in many different genes causing the same disease, with many different mutations in each gene, and with different clinical consequences resulting from the same mutation, even within the same family. for example, mutations in sixteen genes are known to cause adRP and an additional two adRP genes have been mapped but not identified yet (Table 1). This raises two questions: what fraction of adRP cases are accounted for by mutations in known genes, and what accounts for the remaining cases?