Expression of RCAS retrovirus–transferred Arnt and Gdi2 in tumor cells. A, Immunoperoxidase staining of GFP in the cytoplasm of Arnt + Gdi2–induced medulloblastoma cells (arrows) verifying expression of retrovirus-transferred Gdi2. B, Immunostaining of medulloblastoma induced by RCAS-mediated transfer of Myc-tagged Arnt. Monoclonal antibody 9E10 detects expression of retrovirual Arnt protein in the nuclei of tumor cells (arrows). C, Immunoreactive staining for GFP in the cytoplasm of tumor cells (above) attached to the surface of the spinal cord (below) indicating expression of retrovirus-transferred Gdi2. D, Nuclear immunoreactivity for Myc-tagged Arnt in disseminating tumor cells (right) infiltrating the brain stem (left). E, RT-PCR analysis of human Arnt sequences in cerebella from mice injected with RCAS-Shh and RCAS-Arnt (lanes 2–7) and uninjected mouse (lane 1). Reaction products indicate expression of Arnt-Myc fusion protein from the integrated RCAS provirus (lanes 3–6). In two cases (lanes 2 and 7), band intensity of RT-PCR product was not above the background of a control PCR, indicating that retroviral transfer and integration did not lead to transgene expression in all cases. Scale bar, 16 μm ( A, C–D ), 50 μm ( B ).