The identification of low-grade osteosarcomas from OF and FD has become challenging for the same reason.12,16 However, it has been reported that activating mutations in GNAS are highly associated with FD but are absent in OF, providing a molecular tool for their differential diagnosis.5,17 As GNAS mutations can only be detected in 45%–88% of FD cases,18,19 additional methods to better differentiate FD from OF have become vital. Here, GNAS is linked to osteosarcoma.