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People with xeroderma pigmentosum (XP) are extremely sensitive to sunlight, specifically the sun’s ultraviolet (UV) rays. Exposure to UV radiation damages the deoxyribonucleic acid (DNA, the molecule that encodes genetic information) in skin cells and disrupts normal cell functioning. Generally, in healthy people, damaged DNA is repaired by the DNA repair system. However, people with XP have a defective DNA repair system that does not function properly. As a result, unrepaired DNA damage builds up and causes the rapid deterioration observed in the skin of patients with XP, which puts them at very high risk for skin cancer.
Sometimes individuals with XP may also experience neurological complications, such as progressive hearing loss, mental retardation and ataxia (an inability to coordinate muscle movement), among others. This is because the disease also involves the central nervous system (the body system that includes the brain and spinal cord) and causes the premature deterioration of nerve cells in some XP patients. Specifically, the presence of neurological complications in patients with XP correlates with their degree of DNA repair impairment. For instance, patients with the greatest impairment of DNA repair (such as occurs in XPA) are more likely to develop neurodegeneration (deterioration of nerve tissue) and related complications (e.g., ataxia). XP is inherited as an autosomal recessive trait. This means that an abnormal gene on one of the autosomal chromosomes (one of the first 22 "non-sex" chromosomes) from each parent is needed to trigger the disease. People with only one abnormal gene in the gene pair are carriers and can pass on the disease, but because the gene is recessive they do not exhibit the disease. Both parents must be carriers in order for a child to have symptoms of XP. A child who inherits the gene from one parent will only be a carrier of the disease. | 
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