A number of disparate clinical syndromes have been loosely grouped together under the leading of diseases of DNA repair. More logically they should perhaps be termed diseases of diminished capacity to cope with DNA damage, since in only three has defective DNA repair been established as a basis so far. These are xeroderma pigmentosum, ataxia telangiectasia and Fanconi's anaemia. Increased sensitivity may be to radiation, to particular types of chemical mutagens, or to both. This sensitivity may be reflected in an increased liability to chromosome aberrations, decreased cell survival in culture and, in some cases only, increased mutagenesis. In many cases there is an associated increased liability to develop malignant tumours. These syndromes are genetically autosomal recessive so that it is the relatively rare homozygotes which display the full clinical picture. In some cases, however, the heterozygotes share the increased liability to cancer: since these are of relatively high frequency, they may be quantitatively important in the genetics of some human cancers including leukaemias. Immunological abnormalities are common and frequently are selective. This suggests the possibility that the repair systems whose defects are monitored by decreased capacity to cope with DNA damage may also perform functions essential to differentiation during embryonic development. At a molecular level knowledge of the fundamental defects in each of these groups of human mutants is still rudimentary. There is sufficient evidence, however, to conclude that each group is genetically heterogeneous, involving more than one gene locus, so that the total number of genes involved is probably large.
在DNA修复疾病这一主导下，一些不同的临床综合征被松散地归为一类。或许更合理的是，它们应被称为应对DNA损伤能力减弱的疾病，因为到目前为止，仅在三种疾病中确定了有缺陷的DNA修复作为基础。这三种疾病是着色性干皮病、共济失调毛细血管扩张症和范科尼贫血。对辐射、特定类型的化学诱变剂或两者的敏感性可能会增加。这种敏感性可能表现为染色体畸变的易感性增加、培养中细胞存活率降低，并且仅在某些情况下诱变增加。在许多情况下，发生恶性肿瘤的易感性也会增加。这些综合征是常染色体隐性遗传，因此只有相对罕见的纯合子才会表现出完整的临床症状。然而，在某些情况下，杂合子也有患癌易感性增加的情况：由于杂合子频率相对较高，它们在包括白血病在内的一些人类癌症的遗传学中可能在数量上具有重要意义。免疫异常很常见且往往具有选择性。这表明这样一种可能性，即那些通过应对DNA损伤能力下降来监测其缺陷的修复系统，在胚胎发育过程中可能也执行着对分化至关重要的功能。在分子水平上，对这些人类突变体每组中基本缺陷的了解仍然很初步。然而，有足够的证据得出结论，每组在遗传上都是异质的，涉及不止一个基因位点，因此所涉及的基因总数可能很大。
