Earlier experiments showed that a slight change in the chemistry of the viral enzyme makes it inactive and halts the production of new virus particles.

Then, a second viral enzyme, an integrase, incorporates the virus-derived DNA into the genetic material of the infected cell.

Drugs that inhibit viral enzymes generally work for only one type of virus, although there are some exceptions.

The new results are from studies with protease inhibitors, drugs that block a key viral enzyme.

Assembly - RNA and viral enzymes gather at the edge of the cell.

What has made the difference is adding a third drug, usually one that attacks a different viral enzyme, a protein enzyme known as protease.

The protease inhibitor blocks a key viral enzyme called protease, and the older drugs block a different enzyme.

Obvious targets would be the unique viral enzymes, coded for by the genes of the virus.

Acyclovir is a specifically anti-viral drug because the enzymes of normal healthy cells cannot efficiently perform the initial phosphate addition achieved by the viral enzyme.

Integrase is a viral enzyme that integrates retroviral DNA into the host cell genome.