Subependymal giant-cell astrocytoma (SEGA) is a rare tumor associated with tuberous sclerosis complex (TSC). TSC mainly involves the central nervous system (CNS) where SEGA, subependymal nodules, and cortical tubers may be present. First studies suggested the astrocytic nature of SEGA while successive studies demonstrated the mixed glio-neuronal nature. There are similarities between TSC-associated CNS lesions and type IIb focal cortical dysplasia (FCD). In all these pathologies, mammalian target of rapamycin (mTOR) pathway activation has been demonstrated. Recent data evidenced that balloon cells in FCD IIb express glutamine synthetase (GS). GS is involved in the clearance of glutamate. Cells expressing GS might exert an antiepileptic role. We evaluated by immunohistochemistry the glial fibrillary acidic protein (GFAP), neurofilaments (NF), and GS expression and the mTOR status (mTOR and phosphorylated ribosomal protein S6) in 16 SEGAs and 2 cortical tubers. Our purpose was to emphasize the mixed nature of SEGA and to further investigate the similarities between TSC-related CNS lesions (in particular SEGA) and FCD IIb. We confirm the glio-neuronal nature and the common activation of the mTOR pathway in SEGAs. In addition, we report for the first time that these tumors, analogously to FCD IIb, commonly express GS. Notably, the expression of mTOR, phosphorylated ribosomal protein S6, and GS was restricted to gemistocytic-like GFAP-negative cells. GS expression and mTOR pathway activation were also documented in cortical tubers. Further studies are necessary to understand the significance of GS expression in SEGAs as well as in cortical tubers.