The Set1/COMPASS histone methyltransferase complex contains the SET1 histone methyltransferase protein. It functions as a transcriptional co-activator and methylates histone H3 at Lys4 (H3K4). Although first identified in yeast, SET1-related proteins are also found within mammals, including proteins SET1A, SET1B, MLL1, MLL2, MLL3, and MLL4. These proteins work together in a complex to methylate histone H3 at Lys4. In addition, SET1/COMPASS histone methyltransferase complexes are thought to play a role in leukemogenesis due to a significant amount of hematological malignancies containing MLL translocations.