In humans, mutational defects in the "ERCC5"(XPG) gene can cause either the cancer-prone condition xeroderma pigmentosum (XP) alone, or in combination with the severe neurodevelopmental disorder Cockayne syndrome (CS) or the infantile lethal cerebro-oculo-facio-skeletal syndrome. An "ERCC5"(XPG) mutant mouse model presents features of premature aging including cachexia and osteoporosis with pronounced degenerative phenotypes in both liver and brain. These mutant mice develop a multi-system premature aging degenerative phenotype that appears to strengthen the link between DNA damage and aging.(see DNA damage theory of aging).