Ependymosarcomas are rare, biphasic tumors composed of ependymal and sarcomatous components. Due to their rarity, their biologic basis is not well understood. We report the case of a 38-year-old male with anaplastic ependymoma (WHO grade III) that progressed to ependymosarcoma in less than 2 years after multiple resections, chemoradiotherapy, and anti-PD1 immunotherapy. Next-generation sequencing was performed on both high-grade anaplastic ependymoma and ependymosarcoma samples to detect small base changes, insertions, and deletions in exons and splice junctions from a panel of over 400 genes. We identify genetic variants in the tumor suppressors