MLL1 is a histone H3Lys4 methyltransferase and forms a complex with WDR5 and other components.It plays important roles in developmental events,transcriptional regulation,and leukemogenesis.MLL1-fusion proteins resulting from chromosomal translocations are molecular hallmarks of a special type of leukemia,which occurs in over 70% infant leukemia patients and often accompanies poor prognosis.Investigations in the past years on leukemogenesis and the MLL1-WDR5 histone H3Lys4 methyltransferase complex demonstrate that epigenetic regulation is one of the key steps in development and human diseases. MLL1 is a histone H3Lys4 methyltransferase and forms a complex with WDR5 and other components. It plays important roles in developmental events, transcriptional regulation, and leukemogenesis. MLL1-fusion proteins resulting from chromosomal translocations are molecular hallmarks of a special type of leukemia, which occurs in over 70% of infant leukemia patients and often accompanies poor prognosis. Investigations in the past years on leukemogenesis and the MLL1-WDR5 histone H3Lys4 methyltransferase complex demonstrate that epigenetic regulation is one of the key steps in development and human diseases.