Retinal degeneration is the deterioration of the retina caused by the progressive and eventual death of the cells of the retina. There are several reasons for retinal degeneration, including artery or vein occlusion, diabetic retinopathy, R.L.F./R.O.P. (retrolental fibroplasia/ retinopathy of prematurity), or disease (usually hereditary). There are many mechanisms of retinal degeneration attributed to rhodopsin mutations or mutations that involve or affect the function of rhodopsin. One mechanism of retinal degeneration is rhodopsin over expression. Another mechanism, whereby a mutation caused a truncated rhodopsin, was found to affect rod function and increased the rate of photoreceptor degeneration. Mutations in the human rhodopsin that affect its folding, trafficking and activity are the most commonly encountered causes of retinal degeneration in afflicted patients. These may present in many different ways such as impaired vision, night blindness, retinal detachment, light sensitivity, tunnel vision, and loss of peripheral vision to total loss of vision.Renexus (NT-501) consists of encapsulated human cells genetically modified to secrete ciliary neurotropic factor (CNTF). CNTF is a human growth factor that preferentially stimulates and protects neural cells - including photoreceptor cells responsible for detecting light in the retina of the eye. Renexus is designed to continually deliver a safe and therapeutic dose of CNTF into the back of the eye for the treatment of retinal degenerative diseases. Retinitis pigmentosa is a progressive neurodegenerative disorder. RP begins with death of rod photoreceptor cells, which are the only cells in the retina to express rhodopsin and which express it as their most abundant protein. Eventually, loss of rod cells leads to loss of cone cells (cone photoreceptors), the mainstay of human vision.