SHH and neoplasm: To address whether the tumor inhibitory effect of Norrin/Fzd4 signaling extends to models of Shh-MB that do not require Ptch LOH for progression, we used the Neurod2-SmoA1 transgenic mouse, a Shh-MB model where an oncogenic form of Smoothened with an activating point mutation SmoA1 is expressed in GNPs (Hallahan et al., 2004).