A 13-year-old boy presents with a 3-month history of progressive pain and swelling of the left femur. Imaging reveals an aggressive diaphyseal lesion with "onion-skin" periosteal reaction and a large soft tissue mass. Histopathology shows sheets of small round blue cells with scant cytoplasm and round nuclei. CD99 membrane staining is strongly positive. The molecular finding marked **D** in the diagram is identified on FISH analysis. Which of the following best describes the significance of this molecular abnormality in confirming the diagnosis?