A 28-year-old woman presents with progressive pain around the knee for 6 months. X-ray shows an eccentric, lytic, expansile lesion in the distal femur abutting the subchondral plate with a non-sclerotic border. Histopathology reveals sheets of uniform mononuclear stromal cells admixed with numerous osteoclast-like giant cells. The structure marked **B** in the diagram represents the neoplastic component of this lesion. Which of the following molecular alterations is MOST likely responsible for the pathogenesis of this tumor through RANKL-mediated osteoclast recruitment?