Very few familial occurrences of malignant melanomas in human beings have been recorded. The rarity of this occurrence may indicate pure coincidence, and it is possible that genetic factors play no part in the cause of malignant melanomas. However, the presence of the tumor in twins strongly suggests a genetic background.
KATZENELLENBOGEN I, SANDBANK M. Malignant Melanomas in Twins. Arch Dermatol. 1966;94(3):331–332. doi:10.1001/archderm.1966.01600270081017