In different cancer types, including HCC, a decreased level of 5hmC was shown to be caused by either reduced expression of the TET proteins or by inhibition of their activity by toxic metabolite α-hydroxyglutarate which is produced by gain-on-function mutant isocitrate dehydrogenases IDH1 or IDH2 [27, 36]. The gene discussed is IDH1; the disease is hepatocellular carcinoma.