Our lab generated a transgenic mouse line that carries a silent floxed LacZ-stop-Shh transgene, which upon breeding with a mouse that expresses Cre recombinase, the Cre protein excises the LacZ reporter and drives ectopic expression of Sonic hedgehog (Shh), in any tissue that expresses Cre. When bred to Atoh1-Cre mice, the resulting Atoh1-Cre x LoxP-Shh mice had increased and aberrant expression of Shh in the external granule layer (EGL) of the cerebellum, where Atoh1 (Math1) is expressed. We confirmed that the mutant mice did in fact have an increase of Shh indirectly by staining for β-galactosidase activity and directly by measuring Shh transcript and protein expression. We then investigated the effects of the exogenous Shh expression in the cerebellum by comparing the cerebella of the transgenic mice morphologically and histologically to the cerebella of the control mice. We also compared the expression on a marker of proliferation and a marker of differentiation between control and mutant mice at various time points. Postnatal day 6 (P6) and postnatal day 24 (P24) were the principal time point examined. At P6, the main difference in cerebella between control and mutant mice is that the mutant cerebellum is much larger and has more folia. The P24 mutant exhibited many differences in comparison to the P24 control. Namely, in addition to hyperplasia and increased foliation, the mutant P24 cerebellum has a population of cells that remain in the EGL long after they are supposed to migrate inward to establish the internal granule layer (IGL). These cells express the marker of proliferation. Eventually, the mutant mice resulting from the Atoh1-Cre x LoxP-Shh cross develop medulloblastoma-like tumors around postnatal month 4 or 5 (P4 or P5 month). This was confirmed through histological and immunohistochemical data. These data support our hypothesis that the mis-expression of Shh in the cerebellum leads to morphological, histological, and cellular abnormalities.