Given persistent disease and molecular evidence suggesting activation of the PI3K/AKT/mTOR pathway in MCS, maintenance therapy with the mTOR inhibitor sirolimus was initiated.<h4>Conclusion</h4>This case highlights the pivotal role of molecular diagnostics-particularly RNA sequencing-in establishing the diagnosis of mesenchymal chondrosarcoma and differentiating it from other high-grade pediatric sarcomas with overlapping morphology. The gene discussed is AKT1; the disease is mesenchymal chondrosarcoma.