Medically useful drugs whose manufacture involves microorganisms or enzymes produced by microorganisms. Most biotherapeutic agents are produced by genetic engineering, or bioengineering. Because biotherapeutic agents are either identical or similar to proteins produced naturally in the body, they are expected to have less potential to cause side effects than traditional drugs. This is particularly important in cancer therapy. Some dietary supplements are biotherapeutic agents, among them acidophilus and Saccharomyces cerevisiae (brewer’s yeast).