PTCH1 and neoplasm: We have linked the pro-tumor stroma induced by loss of Norrin/Fzd4 signalling to increased GNP proliferation and accelerated Ptch LOH, both events that are known to increase the incidence and decrease latency of Ptch+/− MB (Ayrault et al., 2009; Mille et al., 2014; Pazzaglia et al., 2006a; 2006b; Uziel et al., 2005).