Monozygotic twins are identical individuals born from the fertilization of a single egg cell from a sperm that have the same genetic makeup.

The common elements between two identical twins do not only concern genetics but also involve epigenetics, that is the set of molecular mechanisms that regulate the expression of genes without modifying the DNA. A type of epigenetic modification studied in twins is methylation which consists in the addition of a methyl group (-CH3) to a nitrogenous base.

Through the study of metastable epialleles (ME), epigenetic variants set up in the early embryo and maintained during subsequent cell differentiation, it was possible to observe that the methylation patterns of these genes (ME), responsible for differences in somatic traits in genetically identical mice, they are not random but coincide perfectly in monozygotic twins.

This phenomenon known as "epigenetic supersimilarity" (ESS) it cannot depend on DNA sharing alone. It is unthinkable that two independent individuals can have very similar methylations. The most likely hypothesis is that a series of methylations on these genes occur at an early embryonic stage, before the division of the zygote into two identical copies takes place.

These results focus attention on another issue concerning the onset of tumors. Since aberrant methylations are considered according to some past theories a possible cause of the onset of serious diseases, we wondered if somehow methylations on supersimilar genes could also be associated with the risk of developing tumors.

Se questo venisse confermato bisognerebbe riconsiderare altri fattori oltre l’ereditarietà di alcune malattie nei gemelli.