A DNA repair-deficiency disorder is a medical condition due to reduced functionality of DNA repair. DNA repair defects can cause an accelerated aging disease or an increased risk of cancer, or sometimes both. DNA repair defects and accelerated aging DNA repair defects are seen in nearly all of the diseases described as accelerated aging disease, in which various tissues, organs or systems of the human body age prematurely. Because the accelerated aging diseases display different aspects of aging, but never every aspect, they are often called segmental progerias by biogerontologists. Human disorders with accelerated aging Ataxia-telangiectasia Bloom syndrome Cockayne syndrome Fanconi anemia Progeria (Hutchinson–Gilford progeria syndrome) Rothmund–Thomson syndrome Trichothiodystrophy Werner syndrome Xeroderma pigmentosum Examples Some examples of DNA repair defects causing progeroid syndromes in humans or mice are shown in Table 1. DNA repair defects distinguished from "accelerated aging" Most of the DNA repair deficiency diseases show varying degrees of "accelerated aging" or cancer (often some of both). But elimination of any gene essential for base excision repair kills the embryo—it is too lethal to display symptoms (much less symptoms of cancer or "accelerated aging"). Rothmund-Thomson syndrome and xeroderma pigmentosum display symptoms dominated by vulnerability to cancer, whereas progeria and Werner syndrome show the most features of "accelerated aging". Hereditary nonpolyposis colorectal cancer (HNPCC) is very often caused by a defective MSH2 gene leading to defective mismatch repair, but displays no symptoms of "accelerated aging". On the other hand, Cockayne Syndrome and trichothiodystrophy show mainly features of accelerated aging, but apparently without an increased risk of cancer Some DNA repair defects manifest as neurodegeneration rather than as cancer or "accelerated aging". (Also see the "DNA damage theory of aging" for a discussion of the evidence that DNA damage is the primary underlying cause of aging.) Debate concerning "accelerated aging" Some biogerontologists question that such a thing as "accelerated aging" actually exists, at least partly on the grounds that all of the so-called accelerated aging diseases are segmental progerias. Many disease conditions such as diabetes, high blood pressure, etc., are associated with increased mortality. Without reliable biomarkers of aging it is hard to support the claim that a disease condition represents more than accelerated mortality. Against this position other biogerontologists argue that premature aging phenotypes are identifiable symptoms associated with mechanisms of molecular damage. The fact that these phenotypes are widely recognized justifies classification of the relevant diseases as "accelerated aging".