The gain-of-function IDH1/2 mutations that convert α-ketoglutarate to 2-hydroxyglutarate, a competitive inhibitor for the TET2 enzyme as well as mutual exclusivity of IDH1 and TET2 mutations in certain cancer types, particularly in AML, lead to the widely accepted concept that mutant IDH1 modulates tumorigenesis through the effect on TET2 activity to alter the tumor epigenetic landscape. The gene discussed is TET2; the disease is neoplasm.