Presence of genetically distinct cell lines in the same individual.

Somatic mosaicism – mutation arises from mitotic errors after fertilization and propagates through multiple tissues or organs.

Gonadal mosaicism – mutation only in egg or sperm cells. If parents and relatives do not have the disease, suspect gonadal (or germline mosaicism)

McCune-Albright syndrome – due to mutation affecting G-protein signaling. Presents with unilateral café-au-lait spots with ragged edges, polyostotic fibrous dysplasia (bone is replaced by collagen and fibroblasts), and at least one endocrinopathy (eg, precocious puberty). Lethal if mutation occurs before fertilization (affecting all cells), but survivable in patients with mosaicism.