Fibrous dysplasia is a mosaic disease resulting from post-zygotic activating mutations of the GNAS locus at 20q13.2-q13.3, which codes for the α subunit of the Gs G-coupled protein receptor.[9] In bone, constitutive Gsα signaling results in impaired differentiation and proliferation of bone marrow stromal cells.[10] Proliferation of these cells causes replacement of normal bone and marrow with fibrous tissue.