TET1 was first described in 2003 as a fusion partner of the mixed lineage leukemia (MLL) gene in acute myeloid leukemia (AML; Lorsbach et al., 2003) and 6 years later it was re-discovered as an oxygenase, which can convert 5mC to 5hmC (Tahiliani et al., 2009). Here, KMT2A is linked to acute myeloid leukemia.