ERBB2 and medulloblastoma: To determine whether the high frequency of Treg cells is specific to GBM or is a general characteristic of CNS tumors34 that contributes to their refractoriness to CTL-based therapies, we profiled the composition of T cells infiltrating GBM tumors across several mouse models of brain tumors, including primary adult GBM models (GL261-MGH, CT2A, 005GSC), pediatric brain tumor (group 3 Myc medulloblastoma), and brain metastases from breast cancer (triple-negative and ErbB2+) (Supplementary Fig. 2b).