SHH and medulloblastoma: Whereas previous bulk tumor profiling of medulloblastomas had defined four transcriptomically distinct subgroups of cells within the tumor, including sonic hedgehog (SHH)-activated, wingless-type MMTV integration site family (WNT)-activated, group 3, and group 4 (Thompson et al., 2006; Kool et al., 2008; Northcott et al., 2011), more recent single cell analyses have identified three predominate cell populations.