(MAS) is a disease that affects the bones, skin, and several hormone-producing (endocrine) tissues. It is characterized by replacement of normal bone tissue with areas of abnormal fibrous growth (fibrous dysplasia); patches of abnormal skin coloring with jagged borders (cafe-au-lait spots); and abnormalities in the glands that regulate the body's rate of growth, sexual development, and other metabolic functions (multiple endocrine dysfunction). MAS is caused by a change (mutation) in the GNAS gene that occurs by chance very early in development. As a result, some of the body's cells have a normal version of the GNAS gene, while other cells have the mutated version. This phenomenon is called mosaicism. The severity of MAS and its features depend on the number and location of cells that have the mutated GNAS gene. Because MAS occurs by chance, it is not inherited or passed down from one generation to the next. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.