Familial aggregation of small congenital nevomelanocytic nevi.

We investigated the genetic aspects of congenital nevomelanocytic nevi (CNN) by comparing the prevalence rate of CNN in sibs of probands to that of CNN in newborn infants. Probands included all individuals with small (less than 40 mm) CNN registered photographically during 1982 in a children's hospital dermatology service. A CNN was defined on the basis of gross appearance and presence (according to parents) within the first 2 weeks of life. The 39 probands with small CNN had a total of 65 sibs. Eight of the 65 (12.3%) also had one or more small CNN. This prevalence rate in sibs is 11 times the population-based prevalence rate of CNN in newborn infants (1.1%) based on a published report surveying newborn infants in the same city within the past decade. In five of our 39 study families we also detected an affected parent with CNN. We conclude that small CNN may aggregate in families. Autosomal dominant inheritance with incomplete penetrance or multifactorial determination could account for this observation.