Meet Dorey and Yolanda Nez who had two children with XP. People with XP are extremely sensitive to the damaging effects of ultraviolet radiation. If undiagnosed and untreated, XP can lead to the early onset of skin cancer and blindness. Approximately 25 percent of people with XP develop progressive neurological disease. This is expressed in the form of developmental disabilities and the loss of previously attained abilities, such as walking and talking.