The effect of sun exposure in determining nevus density in UK adolescent twins.

We report a study of 221 teenage twin pairs to examine the genetic and environmental determinants of nevi representing the most potent phenotypic risk factor for melanoma. Our published heritability analysis estimated that nevi are mainly genetically determined. In this paper we examine the role of sun exposure. We report a correlation between nevus density and sun exposure, particularly that acquired in hotter countries than in the UK (mean nevus density 41 per m 2 in those in the highest quartile of exposure vs 24 per m 2 in those with no exposure, p