Fig. 6. Medulloblastoma development is significantly decreased in Ptch1^+/-;Ccnd1^-/- mice compared with Ptch1^+/- mice, despite upregulation of both cyclins D1 and D2 in tumors. Tumors were identified by staining with X-gal (A,B): in the normal cerebellum (A), the staining is strongest in the Purkinje cell layer and inner granule layer, but medulloblastomas (B) were easily identified by their disruption of this pattern resulting in staining throughout the tumor, including at the surface of the brain. (C) Medulloblastomas were also identified histologically. (D-G) Immunohistochemistry of Ptch1^+/- medulloblastomas. (D) In contrast to mature granule cells of the IGL, tumor cells are only weakly positive for the postmitotic marker NeuN. Expression of both cyclins D1 (E) and D2 (G), as well as BrdU incorporation (F) in the tumor but not in adjacent IGL or molecular layers. (H) Ptch1^+/-;Ccnd1^-/- (cycD1 on figure) mice develop significantly fewer medulloblastomas than do Ptch1^+/- mice P=0.0105. (I,J) The medulloblastoma that developed in Ptch1^+/-;Ccnd1^-/- background showed similar patterns of BrdU incorporation (I) and cyclin D2 (J) expression. Mb, medulloblastoma; ML, molecular layer; IGL, inner granule layer. Scale bars: 500 µm in C; 100 µm in D-G,I,J.