The science of how the activity of DNA (deoxyribonucleic acid, which is the fundamental genetic material of cells) can be altered semi-permanently by chemical processes rather than by natural MUTATIONS. Genes contain instructions for making PROTEINS. The natural process of implementing these instructions – gene expression – can be altered by chemical groups attaching themselves to the chemical bases that make up a strand of DNA. This, in turn, affects the generation of proteins from the genes so tagged. Some chemical groups can even stop a gene from being expressed.