DNA in the nuclear environment is tightly packed in structures known as nucleosomes, which consist of DNA wound around histone protein cores. These complexes are important for the regulation of gene expression. Histone methyl transferases, or HMTs, modify histone proteins in order to increase or decrease the accessibility of certain genes. Some cancers may be caused by the overexpression or overactivity of HMTs, which results in the altered expression of genes responsible for cell growth and division.