Melanoma in twins. Cutaneous malignant melanoma in identical twins from a set of triplets

Identical twins and a fraternal triplet were studied because the identical twins developed malignant melanoma at age 53. Each had a preexisting mole, nearly identically placed on the left chest, which began to grow within 2 months of one another. The homozygosity of the twins was established with high probability by appearance, behavior, psychologic testing, deficiency of color vision, red cell typing, haptoglobin typing, dermatoglyphics, lymphocyte typing, mixed lymphocyte culture, lymphocyte transfer, and skin grafting. The concordance for cutaneous malignant melanoma as well as the congruence for the site and identity of age of onset of this tumor in these monozygotic twins clearly indicates the predominant role of genetic rather than environmental factors in the development of their cancers (congruent contemporaneous concordance). Furthermore, the observation suggests that the initial phenomenon eventually leading to malignant melanoma occurred in the conceptus prior to twinning. This finding is relevant to understanding the etiopathogenesis of malignant melanoma in man.