Project Summary Aberrations of chromatin modifying enzymes are frequently found in a wide spectrum of human diseases. Among them, histone H3 K4 methyltransferase mixed lineage leukemia (MLL1, also called KMT2A) has been extensively studied in cancer. MLL1 rearrangement is common in acute pediatric leukemia, in which translocation of one MLL1 allele gives rise to an oncogenic fusion protein. Other MLL1 mutations, including amplification and overexpression, are commonly found in multiple cancers with unknown etiology. MLL1 resides in a multi-subunit complex with conserved components shared among other KMT2 family enzymes, and together they regulate global histone H3K4 methylation. While most studies focus on MLL1’s role in transcription via canonical H3K4 methylation, MLL1 also has non-canonical functions in regulating important cellular processes. We will use multidisciplinary approaches to gain new mechanistic insights on how MLL1 affects cancer development through transcription dependent and independent activities and evaluate its potential as a therapeutic target in hepatocellular carcinoma.