EZH2 and leukemia: Increased H3K27me3 and H3K4me3 depress binding.275 Deletion of MLL3/MLL4 methylases prevents H3K4 methylation and HOXA9-promoted leukemia.274 The primary H3K27me3 “writer” protein, enhancer of zeste homolog 2 (EZH2), is a component of the polycomb repressor complex 2 (PRC2).