Proteins are the workhorses of molecular biology. Enzymes are proteins that perform functional roles. Expression of specific genes in cells produces proteins that give different cell types their unique characteristics. Pancreatic cells, for example, produce insulin to regulate blood sugar levels. Neurons produce neurotransmitters that are essential for brain function, and hemoglobin is made in blood cells to carry oxygen. Examples of enzymes are proteases that break down proteins or enable digestion of food, and polymerases that assemble DNA and mRNA. Regulation of gene expression is essential to ensure appropriate cell function. As mentioned above, cancer results from misregulation of gene expression that causes uncontrolled growth of cells.

Genes are often transduced into bacteria and other cell types to confer the ability to produce specific proteins. These proteins may be extracted for therapeutic use or they can confer new abilities. Genentech introduced the gene for human insulin into bacteria and extracted the resulting protein to produce a treatment for human diabetes. Bacteria have also been modified to perform roles such as decomposing oil spills by adding genes encoding proteins with the ability to break down components of oil.