What is CMN?
A Congenital Melanocytic Naevus (or CMN) is a type of birthmark. It is also a mole.
- Congenital = present at birth
- Melanocytic = to do with melanocytes. Melanocytes are cells in everyone’s skin and hair that produce the pigment melanin. This is what gives us our hair and skin colour. The cells in a CMN look most like melanocytes and they produce pigment, so they are called melanocytic.
- Naevus = birthmark (the plural is naevi).
Though CMN means birthmark or mole at birth, in fact, not all of them are present at birth. Around 1% of CMN appear after birth, usually in the first year of life. They look exactly the same as the ones present at birth, but are called CMN “tardive”, which means “late”.
CMN is now understood as a Mosaic Disorder because CMN is caused by a genetic mutation (change) in a single cell of the baby as it develops during pregnancy. This leads to the baby being born with a mixture of normal cells (not carrying the mutation) and disease cells (carrying the mutation), which is referred to as mosaicism. This event can happen to any child, and importantly is not inherited from either parent. Mosaic disorders affecting the skin are most known because it is possible to see the effects on the skin as birthmarks.
How often do CMN occur?
How often CMN occur depends on the severity of the CMN. Single small CMN are very common, found in 1% of all newborn babies. At the other end of the spectrum, extensive or numerous CMN are much rarer, occurring in around 1 in 10,000-20,000 births. We estimate in the UK that there are around 6000 babies born with a small single CMN, and around 50 babies born per year with the most extensive/numerous CMN. In between these two ends of the spectrum is a spread of how often CMN occur – so for example large single CMN will be more common than the very biggest ones, but less common than the single small ones. So overall we estimate that around 7000 babies in the UK per year are born with some type of CMN. The frequencies seem to be similar around the world and amongst different populations.
Do CMN appear over time?
Very rarely large CMN can appear in the first year or two life, when there was nothing visible at birth. These CMN are called CMN “tardive”, which means “late”.
Some people who have extensive or numerous CMN carrying on developing new small CMN after birth, which is very variable between people and difficult to predict. If you are somebody who develops a lot of new CMN in the first year of life, you may continue to develop new ones for a while. People who don’t develop many in the first year or two of life, don’t seem to suddenly start getting them later. This depends on your own unique pattern, the best predictor we have at the moment if how your naevi have behaved so far.
