Atypical teratoid/rhabdoid tumor (AT/RT) is an uncommon, aggressive, embryonal pediatric brain tumor that almost always develops de novo and does not arise within, or evolve from, other brain tumor types. Although rhabdoid morphology can be seen in other tumor types, these are phenotypic mimics and, with only rare exceptions, do not manifest the INI-1 deletion at the 22q11.2 locus or the INI-1 nuclear protein loss that characterizes AT/RT. A few reports of AT/RT evolving from a low-grade ganglioglioma (GG) or pleomorphic xanthoastrocytoma have appeared. We present the case of a 6-year-old boy with a large right parietal mass whose tumor at initial presentation manifested 2 distinct components: GG with neoplastic neurons, low MIB-1 rate, and retention of INI-1 nuclear immunostaining (immunohistochemical) and, second, AT/RT with rhabdoid cells, polyphenotypic immunohistochemical expression, high MIB-1 rate, and loss of INI-1 nuclear expression. The 2 areas were separately assessed by fluorescence in situ hybridization for monosomy 22; monosomy 22 was identified in the AT/RT component but not in the GG areas. BRAF V600E mutation, a genetic abnormality seen in a significant percentage of pleomorphic xanthoastrocytomas and GGs, was assessed by polymerase chain reaction and identified in the tumor. Dual abnormalities of INI-1 loss and V600E BRAF mutation were identified in a cell culture line established from cerebrospinal fluid metastatic tumor cells. This cell line exhibited extremely rapid growth rate and rhabdoid morphology. Results suggest a postclonal modification in a subset of GG cells, with acquisition of INI-1 loss, confirming by biological methods what was previously suspected in rare reports of AT/RT evolving from other tumor types.