BRCA2 and Mobius syndrome: These included inactivating mutations in the G4-dense gene Ptch1, a most likely primary driver of tumorigenesis in all MBs formed in our model, which is consistent with previous studies of a BRCA2 model of MB (49) and genomic studies of SHH-group MB (11) that also identified PTCH1 mutation as recurrent in pediatric SHH tumors.