The condition in which more than one genetically distinct population of cells coexists within one individual. The term mosaic is used for an individual composed of two or more cell lines of different genetic or chromosomal constitutions, where the cell lines originate from one zygote. Individuals derived from more than one zygote or from more than two gametes are called chimeras. However, because the etiology of mosaicism cannot always be established, the term mosaic often is used to refer to any kind of genetic multiplicity within an individual. Mosaicism is a common phenomenon in both the plant and animal kingdoms; it may originate at any state in the course of ontogeny, and in any tissue in which cells proliferate. Cells with a particular genetic constitution form a clone, and this may appear as a mosaic spot embedded in a background containing cells of different genotype. The several types of mosaicism include chromosomal mosaicism, mosiacism due to mitotic recombination or gene mutation, and functional mosaicism.