BACKGROUND: Genomics has illuminated the extensive intertumoural heterogeneity of medulloblastoma and identified at least four distinct molecular subgroups of the disease. Group 3 and Group 4 subgroup medulloblastomas account for the majority of pediatric cases, yet, oncogenic drivers for these subtypes remain poorly understood. Exome and genome sequencing studies have confirmed a paucity of recurrent gene-level mutations in Group 3 and Group 4, suggesting that alternative oncogenic mechanisms must account for the large fraction of cases that cannot currently be explained by single-nucleotide variants or insertions/deletions alone. METHODS: Analysis of whole-genome sequencing data consisting of 128 primary Group 3 and Group 4 medulloblastoma samples facilitated a systematic, high-resolution screen for chromosomal breakpoints recurrently targeting novel medulloblastoma drivers by structural variation. A non-overlapping set of 22 medulloblastomas was sequenced by long-range paired-end mapping in order to validate structural variants observed in our discovery cohort. Select cases of interest were also investigated at the epigenome-level using a combination of whole-genome bisulphite sequencing and enhancer histone mark ChIP-sequencing. RESULTS: Our systematic analysis of structural variants identified highly disparate genomic structural rearrangements, restricted to Groups 3 and 4, resulting in specific and mutually exclusive activation of the growth factor independent 1 family proto-oncogenes, GFI1 & GFI1B. Diverse mechanisms of structural variation, including duplications, deletions, inversions, translocations, and other complex genomic variants were observed in nearly all GFI1/1B-activated cases. Comprehensive characterization of these structural variants established that GFI1/GFI1B expression becomes activated through relocation of their coding sequences to genomic regions of transcriptionally active chromatin. Functional analyses performed in mice confirmed the oncogenicity of Gfi1/Gfi1b in the context of medulloblastoma and demonstrated apparent synergy between both of these candidates and the c-Myc oncogene. CONCLUSIONS: These studies establish GFI1 and GFI1B as novel, highly prevalent medulloblastoma oncogenes specifically active in Group 3 and Group 4. Given their high frequencies of activation, GFI1 and GFI1B represent excellent candidates for prioritization of molecularly targeted therapy aimed at treatment of a significant proportion of Group 3 and Group 4 medulloblastoma patients. SECONDARY CATEGORY: Pediatrics.