11.   Orlistat, whose brand name is Xenical, blocks an enzyme that is needed to digest fat.

12.   Protease inhibitors block an enzyme that HIV uses to reproduce itself.

13.   So far, however, most drug researchers have tried to look for ways to block enzymes that help build the amyloid.

14.   The drug works by blocking an enzyme that interferes with the blood flow needed to create erections.

15.   The drug, taken an hour before kickoff time, works by blocking an enzyme that interferes with the blood flow needed to create erections.

16.   The drugs work by blocking an enzyme that carves beta-amyloid, a protein fragement, out of a larger molecule called amyoid precursor protein.

17.   The next challenge, experts say, is to find a drug that will block that enzyme.

18.   The new compound is a neuramindase inhibitor, meaning that it blocks an enzyme the flu virus needs to infect cells.

19.   The scientists also say they have found compounds that appear to outsmart the bug by blocking this enzyme.

20.   The traditional anti-HIV drugs such as AZT do so by blocking an enzyme the virus needs to copy its genetic material into the host DNA.