In other cases, the mutational spectrum does not track with a particular embryonic lineage but still exhibits striking tissue specificity; mutations in isocitrate dehydrogenase 1 (IDH1) and IDH2, for example, are found in brain tumors, cholangiocarcinomas, and leukemias but are rare in other tumor types (20). This evidence concerns the gene IDH2 and cholangiocarcinoma.