This effect of UV was more akin to promotion than initiation. This argued that an increase in mutation frequency did not necessarily lead to an increase in cancer. It was, however, based on an observation with one rather poorly growing cell strain and has proved difficult to confirm. From a completely different disease has come evidence that not only supports the above conclusion but extends it. Trichothiodystrophy is a well defined hereditary disease characterised by sulphur-deficient brittle hair usually associated with varying degrees of mental and physical retardation, ichthyosis, dystrophy of the nails, dental caries and cataract.
Many patients are also photosensitive and have been shown to be deficient in excision repair.
Trichothiodystrophy patients, however, show no signs of freckling nor do they appear prone to early skin cancer. It may thus be concluded that not only does an elevated frequency of mutations not necessarily lead to an increase in early skin cancer but that a defect in excision repair is also insufficient to lead to early skin cancer. The suggestion that a defect in immune surveillance is the reason for the early appearance of tumors is not proven, but the evidence is increasingly compelling. They did not, however, find that delayed hypersensitivity responses were generally depressed in skin tests for a variety of recall antigens. Two recent studies illustrate this.