Proper regulation of gene expression is essential. Under- or over-expression of genes can have deleterious effects. Cancer is caused by misregulation of gene expression that results in uncontrolled cell growth. A solution for genes that are under-expressed is to use gene therapy to spur additional production. One of the challenges of gene therapy is developing methods to regulate the expression of genes that are introduced into cells and ensure that they are not over-expressed. A solution for diseases caused by over-expressed genes is RNA interference. This procedure prevents translation of mRNA, inhibiting protein production. RNA interference is discussed in further detail in Chapter 6.

Messenger RNA (mRNA) is used to relay information from genes in DNA to the protein synthesis machinery. An additional feature of mRNA is that it can be destroyed once sufficient protein is produced, permitting an extra level of control of gene expression. RNA is also present in forms other than mRNA, but these are beyond the scope of this discussion. It is possible to affect expression of genes by targeting their mRNA with antisense RNA or DNA. Unlike double-stranded DNA, mRNA is single stranded. Nucleic acids (DNA or RNA) containing a sequence that can bind a given mRNA will prevent translation by the protein synthesis machinery, inhibiting gene expression. The Flavr Savr tomato, a tomato engineered to have a long shelf life, was produced by introducing antisense RNA corresponding to mRNA for an enzyme involved in fruit spoilage. By inhibiting expression of this gene, fruit spoilage was delayed. In 1998 the FDA approved Isis Pharmaceuticals' Vitravene, the first antisense drug, to treat cytomegalovirus-induced retinitis.

Just as DNA and RNA are composed of linked nucleotide building blocks, proteins are comprised of linked chains of amino acid units. When mRNA is translated to produce a protein, the protein-synthesis machinery 'reads' the nucleotides three at a time, assembling amino acid chains that correspond to the mRNA sequence. The basic elements of the protein-synthetic machinery are tRNA, a form of RNA that transfers amino acids and ribosomes, which help form the bonds that attach amino acids in a protein chain.