*8.4. PRCD*

Progressive rod–cone degeneration (*Prcd*) was a formerly unknown gene that was mapped and sequenced first in dogs with progressive rod–cone degeneration (PRCD) PRA [172]. Dogs with this form of PRA had been studied in detail over many years and many di fferent breeds of dog are a ffected. The mutation is p.Cys2Tyr, which is in a highly conserved region of the gene. An identical mutation was found in a human RP patient. Additional mutations also located in exon 1 of *PRCD* have been identified in RP patients [172,173]. Since the discovery of *Prcd*, a mouse knockout model has been generated and experiments have shown Prcd's involvement in photoreceptor disc formation and maintenance [174–176]. Studies in the PRCD dog showed an early ultrastructural change that may have resulted from this purported function. This was the development of vesicular profiles adjacent to outer segments being consistent with abnormality in disc formation, furthermore studies also showed affected dogs had slower photoreceptor disc renewal than normal controls [177].
