FUS is a gene that makes a protein involved in reading and processing the cell’s genetic instructions. Its name stands for “fused in sarcoma” because it was first discovered joined to another gene in a type of sarcoma. In some tumors, FUS breaks and joins with a different gene, creating an abnormal fusion gene. Testing for FUS helps pathologists diagnose certain sarcomas, especially myxoid liposarcoma and low-grade fibromyxoid sarcoma.
A FUS result is a test result, not a diagnosis. This article explains what FUS does, which tumors have FUS changes, and what a FUS result in your pathology report means.
Inherited cFUS changes are also a rare cause of amyotrophic lateral sclerosis (ALS), a disease of the nerves that control muscles. That condition differs from the FUS fusions found in tumors, which this article describes.
FUS sits on chromosome 16. It belongs to the same gene family as EWSR1, and the two genes behave similarly. In certain tumors, a break occurs in FUS and in another gene, and the two broken pieces join together. This event is called a translocation. The fusion gene makes an abnormal protein that drives tumor growth.
The change happens only in the tumor cells. It is not inherited and cannot be passed on to children.
FUS fusions are found in a small number of tumor types. Each partner gene is linked to a particular tumor:
Pathologists test for FUS changes using molecular tests on the tumor tissue. Fluorescence in situ hybridization (FISH) can show that the FUS gene has been broken apart. Some laboratories instead test for the DDIT3 gene, which is broken in almost every myxoid liposarcoma. Next-generation sequencing (NGS) can identify both FUS and its exact partner gene.
A positive result means the tumor cells have a FUS rearrangement or fusion. Your report may say “FUS rearrangement detected” or name the specific fusion, such as “FUS::DDIT3 fusion.” The pathologist interprets the result together with what the tumor looks like under the microscope and the results of other tests. A fusion with a known partner gene usually confirms a specific diagnosis.
A negative result means no FUS rearrangement was found. It makes some diagnoses less likely but does not rule them out, because some of these tumors have a fusion involving EWSR1 or another gene instead.