We know now that the body can produce such substances, but it was a long jump from theory to proof.

So far, genetic engineering has mainly been used to produce substances that already exist in nature.

I have produced a substance that is similar to radium.

Many nuclear power plants produce such substances; usually they are processed to radioactive waste.

This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome.

Perhaps cancer cells produced a substance that enticed blood vessels to grow toward them.

Animals are being genetically transformed so that they can produce drugs and other useful substances in their milk.

They produce helpful substances like penicillin, but can also cause damage to plants and humans.

Researchers have found that one function of the brain is to produce substances that can improve your health.

The xenotransplantation of non-human cells that are used to produce a needed substance.