Epigenetics refers to how chromatin-associated factors and reversible chromatin modifications maintain DNA-based programs by regulating the chromatin structure (Strahl and Allis,2000).In recent years,genome sequencing studies of cancer have shown that genes encoding epigenetic factors are commonly mutated in cancer (Garraway and Lander,2013).Dysregulated,or mutant,epigenetic factors influence tumorigenesis progression (Dawson and Kouzarides,2012).Disruptor of telomeric silencing 1 (Dot1) and its mammalian homolog,DOT1-like (DOT1L),the non-SET domain-containing methyltransferases,target histone H3K79 for monomethylation,dimethylation,and trimethylation (Feng et al.,2002).Analyses of mouse,fly,and human genomes showed that H3K79me2 is an active transcription marker (Nguyen and Zhang,2011).The mechanisms underlying H3K79me2-mediated transcriptional activation and the roles of DOT1L in tumor progression require further exploration.