Identical twins remain genetically identical throughout life.

Identical twins are called monozygotic because they begin when one fertilized egg gives rise to two developing embryos.

That shared origin means they start with almost the same inherited DNA, which is why identical twins have been so useful for separating genetic and environmental influences.

But DNA is copied every time cells divide, and copying is not perfectly error-free.

If a new mutation occurs before the embryo splits, both twins may inherit it. If it occurs afterward, the change can end up in only one twin. A later mutation may even occur in only some of that twin’s cells, producing what is called mosaicism.

Modern genome sequencing can detect these small differences. Researchers have found single-letter DNA changes as well as larger alterations, including gains or losses of DNA segments, that are present in one identical twin but not the other.

The differences are usually tiny compared with the enormous amount of DNA the twins still share. Identical twins remain much more genetically similar than ordinary siblings.

Environmental and epigenetic differences can also make twins diverge, but those are separate from changes in the DNA sequence itself. The correction is simply that “identical” does not mean every cell in both people must retain a perfectly identical genome for life.