SHH and medulloblastoma: We reported previously that ectopic expression of genes encoding Eras (embryonic stem cell–expressed Ras), Lhx1 (LIM-class homeobox gene 1), Ccrk (cell cycle–related kinase), and the phosphatidyl inositol 3-kinase signaling protein Akt shifted the growth characteristics of Shh-induced medulloblastoma from a localized pattern to one in which tumor cells seeded the leptomeninges of the brain and spinal cord [18].