Mutations in the "ERCC2"(XPD) gene can lead to various syndromes, either xeroderma pigmentosum (XP), trichothiodystrophy (TTD) or a combination of XP and TTD (XPTTD), or a combination of XP and Cockayne syndrome (XPCS). TTD and CS both display features of premature aging. These features may include sensorineural deafness, retinal degeneration, white matter hypomethylation, central nervous system calcification, reduced stature, and cachexia (loss of subcutaneous fat tissue). XPCS and TTD fibroblasts from "ERCC2"(XPD) mutant human and mouse show evidence of defective repair of oxidative DNA damages that may underlie the segmental progeroid (premature aging) symptoms (see DNA damage theory of aging).