Medulloblastoma cells (MBC) were obtained from SmoA1 mouse medulloblastomas. Briefly, tumors were disassociated and cells were incubated in Trypsin/EDTA solution for 30 minutes, then passed through a cell strainer. Cells were subsequently separated on a density step gradient of 35% and 60% Percoll solution (Sigma). Purified MBCs were enriched by pre-plating on uncoated tissue culture dishes to remove adherent fibroblasts and glial cells. Non-adherent cells were plated on tissue culture dishes pre-coated with poly D-lysine (Sigma) and Matrigel (BD Biosciences), infected with retroviruses 24 hours later, and cultured for 24 more hours before implantation or irradiation.