Progressive osseous heteroplasia. In this rare disorder, bone forms within skin and muscle tissue. Ectopic bone formation starts in the dermis and subcutaneous fat and gradually extends into other tissues such as tendons and skeletal muscle. Progressive osseous heteroplasia is caused by a mutation in the GNAS gene.

Source: Prose NS, Kristal L. Weinberg's Color Atlas of Pediatric Dermatology, 5e; 2017.