Illustration of a small-molecule inhibitor bound to the embryonic ectoderm development (EED) subunit of the histone methyltransferase polycomb repressive complex 2 (PRC2). Histone methyltransferases transfer methyl groups to amino acid residues on histones, proteins that play an important role in compacting DNA (deoxyribonucleic acid). This process is known as methylation. Histone methylation can activate or repress the transcription of genes without changing the actual genetic sequence and so is an example of an epigenetic change. Aberrant activity of PRC2 has been linked to several cancers and inhibitors of the enzyme are being developed as anticancer drugs.