The multiple genetic hit model of cancer predicts that normal individuals should have stable populations of cancer-prone, but noncancerous, mutant cells awaiting further genetic hits. We report that whole-mount preparations of human skin contain clonal patches of p53-mutated keratinocytes, arising from the dermal-epidermal junction and from hair follicles. These clones, 60-3000 cells in size, are present at frequencies exceeding 40 cells per cm 2 and together involve as much as 4% of the epidermis. In sun-exposed skin, clones are both more frequent and larger than in sun-shielded skin. We conclude that, in addition to being a tumorigenic mutagen, sunlight acts as a tumor promoter by favoring the clonal expansion of p53-mutated cells. These combined actions of sunlight result in normal individuals carrying a substantial burden of keratinocytes predisposed to cancer.Although skin cancers typically arise in patients aged 50-70, epidemiologic evidence indicates that much of the critical sunlight exposure is received before the age of 18 (1, 2). This early role of sunlight is supported by the finding of sunlightinduced mutations in the p53 tumor suppressor gene in actinic keratosis, the precancerous lesion for squamous cell carcinoma of the skin. In addition, p53-mutated cells are present in skin flanking human tumors and in UV-irradiated mouse skin (3-8). Mutations at particular p53 codons are present in sun-exposed normal human skin at frequencies of 10 Ϫ6 to 10 Ϫ2 (5, 9). Other human tumors for which mutation of p53 appears to be an early event include head and neck cancer and hepatocellular carcinoma (10, 11).Because keratinocytes are continuously lost through squamous differentiation and desquamation, it seems likely that the cell targeted by sunlight decades before a tumor's appearance is a stem cell. If so, the keratinocytes containing the mutations measured in normal skin would not be randomly dispersed but instead would reside in clonal patches arising from mutated stem cells. The frequency of p53 mutations measured in a biopsy of normal skin would then depend on whether the biopsy included a clone. The spatial arrangement of the cells in the clone might give clues to the geometry of early carcinogenesis, including the site of the stem cells from which skin tumors originate in humans.We therefore devised a whole-mount preparation method for human epidermis that permitted immunohistochemical analysis for stabilized p53 protein. A p53 mutation usually leads to nuclear immunopositivity (12). A large enough sample of skin might contain rare patches staining intensely for p53. We report here that such patches are not only present and contain mutations but are also frequent, indicating the existence of a large population of cells in normal skin that are presumably predisposed to skin cancer. In addition, we present evidence that sunlight can act both as a tumor initiator and as a tumor promoter for p53-mutated cells. MATERIALS AND METHODSEpidermal Whole Mounts. Fresh skin samples were obtained from discard...

The combination of hard palate spacer grafting, lateral canthoplasty, and midface suspension is an effective, aesthetic, and functional treatment for moderate to severe lower eyelid retraction resulting from multiple causes. This tripartite procedure is associated with predictable results, a low morbidity rate, and high patient satisfaction.