In healthy people all the cells in the body (somatic cells) have the same chromosomes and same genes. Mosaicism is a condition in which cells within the same person have a different genetic makeup. It is caused by a problem occurring during cell division leading to two genetically different types of cells forming and then going on to propagate themselves. It most often occurs very early in the development of the unborn baby. If it causes disease then the disease is often milder than in people who have an abnormal mutation in all the cells in their body.