A mosaic is formed by two or more genetically different populations of cells originating from a single ‘healthy’ wild-type zygote. It is developed from an early-stage embryonic mutation, called ‘somatic’. Mosaics are well known in biology, especially in plants and fruits and animals (insects, e.g. Drosophila species) but, in contrast, in humans they represent recent knowledge and even more recent understanding. From 1983 onward, many molecular biology studies have attempted to explain the pathogenesis of this phenomenon.