Molecular genetics: Fibrous dysplasia results from somatic gain-of-function mutation in GNAS1 gene on chromosome 20q13. It encodes for alpha-stimulatory subunit of heterodimeric G proteins. This leads to constitutively active G protein that causes cellular proliferation. The phenotype depends upon the post-zygotic stage of development at which the mutation is acquired. If acquired during early embryogenesis, it produces McCune-Albright syndrome. If the mutation occurs in an osteoblast precursor in postnatal life, it causes monostotic fibrous dysplasia. The image shows irregular bony trabeculae in a background of fibroblastic stroma. Nodules of hyaline cartilage are also present - a feature seen in about 10% of cases of fibrous dysplasia.