GNAS and myxofibrosarcoma: Carbohydrate antigen 19-9, a tumor marker, has been reported to be correlated with intramuscular myxoma; however, it may also increase in multiple other malignant and benign conditions.16 Next-generation sequencing for determining GNAS mutation status on small core biopsies can facilitate the diagnosis of intramuscular myxoma and differentiate it from other tumors such as low-grade myxofibrosarcomas.17,18