Alternative factors that could cause pathway activation in CNS PNETs include inactivating mutations in the pathway inhibitors AXIN1 and AXIN2. Mutations have been identified in both genes in different tumour types including medulloblastoma, hepatocellular and colon carcinoma (Liu et al, 2000; Satoh et al, 2000; Taniguchi et al, 2002; Baeza et al, 2003; Koch et al, 2007). The gene discussed is AXIN2; the disease is medulloblastoma.