Histone H3-lysine79 (H3K79) methyltransferase DOT1L plays critical roles in 
normal cell differentiation as well as initiation of acute leukemia. We used 
structure- and mechanism-based design to discover several potent inhibitors of 
DOT1L with IC(50) values as low as 38 nM. These inhibitors exhibit only weak or 
no activities against four other representative histone lysine and arginine 
methyltransferases, G9a, SUV39H1, PRMT1 and CARM1. The X-ray crystal structure 
of a DOT1L-inhibitor complex reveals that the N6-methyl group of the inhibitor, 
located favorably in a predominantly hydrophobic cavity of DOT1L, provides the 
observed high selectivity. Structural analysis shows that it will disrupt at 
least one H-bond and/or have steric repulsion for other histone 
methyltransferases. These compounds represent novel chemical probes for 
biological function studies of DOT1L in health and disease.