Fig. 2.

Mutation timing determines the extent of the disease and clinical manifestations. The stage of embryogenesis during which a mutation occurs, and the locations to where mutated progenitors subsequently migrate, determines if a patient will have a single lesion, polyostotic disease or one of the FD-related syndromes. Mutations that occur at early stages of embryogenesis result in the widespread distribution of the lesions. Mutations that develop at late stages of embryogenesis lead to more focused distribution of the lesions. Patients with McCune-Albright syndrome (MAS) may have different extra-skeletal abnormalities. Some of these abnormalities may progress to malignancy; part of them become stable throughout life; some abnormalities can regress or disappear